Elongated photovoltaic cells in tubular casings

ABSTRACT

A solar cell unit comprising a cylindrical shaped solar cell and a transparent tubular casing is provided. The tubular shaped solar cell comprises a back-electrode, a semiconductor junction circumferentially disposed on the back-electrode and a transparent conductive layer disposed on the semiconductor junction. The transparent tubular casing is circumferentially sealed onto the transparent conductive layer of the cylindrical shaped solar cell. A solar cell unit comprising a cylindrical shaped solar cell, a filler layer, and a transparent tubular casing is provided. The cylindrical shaped solar cell comprises a cylindrical substrate, a back-electrode circumferentially disposed on the cylindrical substrate, a semiconductor junction circumferentially disposed on the back-electrode, and a transparent conductive layer disposed on the semiconductor junction. The filler layer is circumferentially disposed on the transparent conductive layer and the transparent tubular casing is circumferentially disposed onto the filler layer.

1. FIELD OF THE INVENTION

This invention relates to solar cell assemblies for converting solarenergy into electrical energy and more particularly to improved solarcell assemblies.

2. BACKGROUND OF THE INVENTION

Solar cells are typically fabricated as separate physical entities withlight gathering surface areas on the order of 4-6 cm² or larger. Forthis reason, it is standard practice for power generating applicationsto mount the cells in a flat array on a supporting substrate or panel sothat their light gathering surfaces provide an approximation of a singlelarge light gathering surface. Also, since each cell itself generatesonly a small amount of power, the required voltage and/or current isrealized by interconnecting the cells of the array in a series and/orparallel matrix.

A conventional prior art solar cell structure is shown in FIG. 1.Because of the large range in the thickness of the different layers,they are depicted schematically. Moreover, FIG. 1 is highly schematic sothat it represents the features of both “thick-film” solar cells and“thin-film” solar cells. In general, solar cells that use an indirectband gap material to absorb light are typically configured as“thick-film” solar cells because a thick film of the absorber layer isrequired to absorb a sufficient amount of light. Solar cells that use adirect band gap material to absorb light are typically configured as“thin-film” solar cells because only a thin layer of the direct band-gapmaterial is needed to absorb a sufficient amount of light.

The arrows at the top of FIG. 1 show the source of direct solarillumination on the cell. Layer 102 is the substrate. Glass or metal isa common substrate. In thin-film solar cells, substrate 102 can be-apolymer-based backing, metal, or glass. In some instances, there is anencapsulation layer (not shown) coating substrate 102. Layer 104 is theback electrical contact for the solar cell.

Layer 106 is the semiconductor absorber layer. Back electrical contact104 makes ohmic contact with absorber layer 106. In many but not allcases, absorber layer 106 is a p-type semiconductor. Absorber layer 106is thick enough to absorb light. Layer 108 is the semiconductor junctionpartner-that, together with semiconductor absorber layer 106, completesthe formation of a p-n junction. A p-n junction is a common type ofjunction found in solar cells. In p-n junction based solar cells, whensemiconductor absorber layer 106 is a p-type doped material, junctionpartner 108 is an n-type doped material. Conversely, when semiconductorabsorber layer 106 is an n-type doped material, junction partner 108 isa p-type doped material. Generally, junction partner 108 is much thinnerthan absorber layer 106. For example, in some instances junction partner108 has a thickness of about 0.05 microns. Junction partner 108 ishighly transparent to solar radiation. Junction partner 108 is alsoknown as the window layer, since it lets the light pass down to absorberlayer 106.

In a typical thick-film solar cell, absorber layer 106 and window layer108 can be made from the same semiconductor material but have differentcarrier types (dopants) and/or carrier concentrations in order to givethe two layers their distinct p-type and n-type properties. In thin-filmsolar cells in which copper-indium-gallium-diselenide (CIGS) is theabsorber layer 106, the use of CdS to form junction partner 108 hasresulted in high efficiency cells. Other materials that can be used forjunction partner 108 include, but are not limited to, In₂Se₃, In₂S₃,ZnS, ZnSe, CdlnS, CdZnS, ZnIn₂Se₄, Zn_(1-x)Mg_(x)O, CdS, SnO₂, ZnO, ZrO₂and doped ZnO.

Layer 110 is the counter electrode, which completes the functioningcell. Counter electrode 110 is used to draw current away from thejunction since junction partner 108 is generally too resistive to servethis function. As such, counter electrode 110 should be highlyconductive and transparent to light. Counter electrode 110 can in factbe a comb-like structure of metal printed onto layer 108 rather thanforming a discrete layer. Counter electrode 110 is typically atransparent conductive oxide (TCO) such as doped zinc oxide (e.g.,aluminum doped zinc oxide, gallium doped zinc oxide, boron dope zincoxide), indium-tin-oxide (ITO), tin oxide (SnO₂), or indium-zinc oxide.However, even when a TCO layer is present, a bus bar network 114 istypically needed in conventional solar cells to draw off current sincethe TCO has too much resistance to efficiently perform this function inlarger solar cells. Network 114 shortens the distance charge carriersmust move in the TCO layer in order to reach the metal contact, therebyreducing resistive losses. The metal bus bars, also termed grid lines,can be made of any reasonably conductive metal such as, for example,silver, steel or aluminum. In the design of network 114, there is designa trade off between thicker grid lines that are more electricallyconductive but block more light, and thin grid lines that are lesselectrically conductive but block less light. The metal bars arepreferably configured in a comb-like arrangement to permit light raysthrough TCO layer 110. Bus bar network layer 114 and TCO layer 110,combined, act as a single metallurgical unit, functionally interfacingwith a first ohmic contact to form a current collection circuit. In U.S.Pat. No. 6,548,751 to Sverdrup et al., hereby incorporated by referenceherein in its entirety, a combined silver bus bar network andindium-tin-oxide layer function as a single, transparent ITO/Ag layer.

Layer 112 is an antireflective coating that can allow a significantamount of extra light into the cell. Depending on the intended use ofthe cell, it might be deposited directly on the top conductor asillustrated in FIG. 1. Alternatively or additionally, antireflectivecoating 112 made be deposited on a separate cover glass that overlaystop electrode 110. Ideally, the antireflective coating reduces thereflection of the cell to very near zero over the spectral region inwhich photoelectric absorption occurs, and at the same time increasesthe reflection in the other spectral regions to reduce heating. U.S.Pat. No. 6,107,564 to Aguilera et al., hereby incorporated by referenceherein in its entirety, describes representative antireflective coatingsthat are known in the art.

Solar cells typically produce only a small voltage. For example, siliconbased solar cells produce a voltage of about 0.6 volts (V). Thus, solarcells are interconnected in series or parallel in order to achievegreater voltages. When connected in series, voltages of individual cellsadd together while current remains the same. Thus, solar cells arrangedin series reduce the amount of current flow through such cells, comparedto analogous solar cells arrange in parallel, thereby improvingefficiency. As illustrated in FIG. 1, the arrangement of solar cells inseries is accomplished using interconnects 116. In general, aninterconnect 116 places the first electrode of one solar cell inelectrical communication with the counter-electrode of an adjoiningsolar cell.

As noted above and as illustrated in FIG. 1, conventional solar cellsare typically in the form of a plate structure. Although such cells arehighly efficient when they are smaller, larger planar solar cells havereduced efficiency because it is harder to make the semiconductor filmsthat form the junction in such solar cells uniform. Furthermore, theoccurrence of pinholes and similar flaws increase in larger planar solarcells. These features can cause shunts across the junction.

A number of problems are associated with solar cell designs present inthe known art. A number of prior art solar cell designs and some of thedisadvantages of each design will now be discussed.

As illustrated in FIG. 2A, U.S. Pat. No. 6,762,359 B2 to Asia et al.discloses a solar cell 210 including a p-type layer 12 and an n-typelayer 14. A first electrode 32 is provided on one side of the solarcell. Electrode 32 is in electrical contact with n-type layer 14 ofsolar cell 210. Second electrode 60 is on the opposing side of the solarcell. Electrode 60 is in electrical contact with the p-type layer of thesolar cell. Light-transmitting layers 200 and 202 form one side ofdevice 210 while layer 62 forms the other side. Electrodes 32 and 60 areseparated by insulators 40 and 50. In some instances, the solar cell hasa tubular shape rather than the spherical shape illustrated in FIG. 2.While device 210 is functional, it is unsatisfactory. Electrode 60 hasto pierce absorber 12 in order to make an electrical contact. Thisresults in a net loss in absorber area, making the solar cell lessefficient. Furthermore, such a junction is difficult to make relative toother solar cell designs.

As illustrated in FIG. 2B, U.S. Pat. No. 3,976,508 to Mlavsky disclosesa tubular solar cell comprising a cylindrical silicon tube 2 of n-typeconductivity that has been subjected to diffusion of boron into itsouter surface to form an outer p-conductivity type region 4 and thus ap-n junction 6. The inner surface of the cylindrical tube is providedwith a first electrode in the form of an adherent metal conductive film8 that forms an ohmic contact with the tube. Film 8 covers the entireinner surface of the tube and consists of a selected metal or metalalloy having relatively high conductivity, e.g., gold, nickel, aluminum,copper or the like, as disclosed in U.S. Pat. Nos. 2,984,775, 3,046,324and 3,005,862. The outer surface is provided with a second electrode inthe form of a grid consisting of a plurality of circumferentiallyextending conductors 10 that are connected together by one or morelongitudinally-extending conductors 12. The opposite ends of the outersurface of the hollow tube are provided with twocircumferentially-extending terminal conductors 14 and 16 that interceptthe longitudinally-extending conductors 12. The spacing of thecircumferentially-extending conductors 10 and thelongitudinally-extending conductors 12 is such as to leave areas 18 ofthe outer surface of the tube exposed to solar radiation. Conductors 12,14 and 16 are made wider than the circumferentially-extending conductors10 since they carry a greater current than any of the latter. Theseconductors are made of an adherent metal film like the inner electrode 8and form ohmic contacts with the outer surface of the tube. While thesolar cell disclosed in FIG. 2 is functional, it is also unsatisfactory.Conductors 12, 14, and 16 are not transparent to light and therefore theamount of light that the solar cell receives is reduced.

U.S. Pat. No. 3,990,914 to Weinstein and Lee discloses another form oftubular solar cell. Like Mlavsky, the Weinsten and Lee solar cell has ahollow core. However, unlike Mlavsky, Weinstein and Lee dispose thesolar cell on a glass tubular support member. The Weinstein and Leesolar cell has the drawback of being bulky and expensive to build.

Referring to FIGS. 2C and 2D, Japanese Patent Application KokaiPublication Number S59-125670, Toppan Printing Company, published Jul.20, 1984 (hereinafter “S59-125670”) discloses a rod-shaped solar cell.The rod shaped solar cell is depicted in cross-section in FIG. 2C. Aconducting metal is used as core 1 of the cell. A light-activatedamorphous silicon semiconductor layer 3 is provided on core 1. Anelectrically conductive transparent conductive layer 4 is built up ontop of semiconductor layer 3. The transparent conductive layer 4 can bemade of materials such as indium oxide, tin oxide or indium tin oxide(ITO) and the like. As illustrated in FIG. 2C, a layer 5, made of a goodelectrical conductor, is provided on the lower portion of the solarcell. The publication states that this good conductive layer 5 is notparticularly necessary but helps to lower the contact resistance betweenthe rod and a conductive substrate 7 that serves as a counter-electrode.As such, conductive layer 5 serves as a current collector thatsupplements the conductivity of counter-electrode 7 illustrated in FIG.2D.

As illustrated in FIG. 2D, rod-shaped solar cells 6 are multiplyarranged in a row parallel with each other, and counter-electrode layer7 is provided on the surface of the rods that is not irradiated by lightso as to electrically make contact with each transparent conductivelayer 4. The rod-shaped solar cells 6 are arranged in parallel and bothends of the solar cells are hardened with resin or a similar material inorder to fix the rods in place.

S59-125670 addresses many of the drawbacks associated with planar solarcells. However, S59-125670 has a number of significant drawbacks thatlimit the efficiency of the disclosed devices. First, the manner inwhich current is drawn off the exterior surface is inefficient becauselayer 5 does not wrap all the way around the rod (e.g., see FIG. 2C).Second, substrate 7 is a metal plate that does not permit the passage oflight. Thus, a full side of each rod is not exposed to light and canthus serve as a leakage path. Such a leakage path reduces the efficiencyof the solar cell. For example, any such dark junction areas will resultin a leakage that will detract from the photocurrent of the cell.Another disadvantage with the design disclosed in FIGS. 2C and 2D isthat the rods are arranged in parallel rather than in series. Thus, thecurrent levels in such devices will be large, relative to acorresponding serially arranged model, and therefore subject toresistive losses.

Referring to FIG. 2E, German Unexamined Patent Application DE 43 39 547A1 to Twin Solar-Technik Entwicklungs-GmbH, published May 24, 1995,(hereinafter “Twin Solar”) also discloses a plurality of rod-shapedsolar cells 2 arranged in a parallel manner inside a transparent sheet28, which forms the body of the solar cell. Thus, Twin Solar does nothave some of the drawbacks found in S59-125670. Transparent sheet 28allows light in from both faces 47A and 47B. Transparent sheet 28 isinstalled at a distance from a wall 27 in such a manner as to provide anair gap 26 through which liquid coolant can flow. Thus, Twin Solardevices have the drawback that they are not truly bifacial. In otherwords, only face 47A of the Twin Solar device is capable of receivingdirect light. As defined here, “direct light” is light that has notpassed through any media other than air. For example, light that haspassed through a transparent substrate, into a solar cell assembly andexited the assembly, is no longer direct light once it exits the solarcell assembly. Light that has merely reflected off of a surface,however, is direct light provided that it has not passed through a solarcell assembly. Under this definition of direct light, face 47B is notconfigured to receive direct light. This is because all light receivedby face 47B must first traverse the body of the solar cell apparatusafter entering the solar cell apparatus through face 47A. Such lightmust then traverse cooling chamber 26, reflect off back wall 42, andfinally re-enter the solar cell through face 47B. The solar cellassembly is therefore inefficient because direct light cannot enter bothsides of the assembly.

Although tubular designs of solar cells have addressed many of thedrawbacks associated with planar solar cells, some problems remainunresolved. The capacity of solar cells to withstand physical shock isone unresolved problem. Conventional solar cell panels often crack aftera certain number of years, often even before the gained energy benefitcan balance their production costs. Solar cell assemblies are oftenbuilt from small individual solar cell units. This approach providesefficiency and flexibility. Smaller solar cells are easier tomanufacture at a large scale, and they can also be assembled intodifferent sizes and shapes to suit the ultimate application. Inevitably,the smaller solar cell unit design also comes with the price offragility. The smaller solar cell units easily break under pressureduring transportation or routine handling processes. What are needed inthe art are methods and systems that provide support and strength tosolar cell units while maintaining the advantages of the small design.

Discussion or citation of a reference herein will not be construed as anadmission that such reference is prior art to the present invention.

3. SUMMARY OF THE INVENTION

One aspect of the invention provides a solar cell unit comprising acylindrical shaped solar cell and a transparent tubular casing. Thecylindrical shaped solar cell comprises a back-electrode, asemiconductor junction layer circumferentially disposed on theback-electrode, and a transparent conductive layer disposed on thesemiconductor junction. The transparent tubular casing iscircumferentially sealed onto the cylindrically shaped solar cell sothat there is no air between the transparent tubular casing and thecylindrically shaped solar cell in the solar cell unit. In someembodiments, the transparent tubular casing is made of plastic or glass.In some embodiments, the cylindrically shaped solar cell furthercomprises a cylindrical substrate and the back-electrode iscircumferentially disposed on the cylindrical substrate. The cylindricalsubstrate can be made of a wide variety of materials including plastic,metal, or glass. Typically, the cylindrical substrate is hollowed (e.g.,a tube). Therefore, fluids such as air, nitrogen, or helium can bepassed through the cylindrical substrate in many embodiments of thepresent invention. In some embodiments, however, the cylindricalsubstrate is solid.

In some embodiments, the semiconductor junction comprises ahomojunction, a heterojunction, a heteroface junction, a buriedhomojunction, a p-i-n junction, or a tandem junction. In someembodiments, the conductor junction comprises an absorber layer and ajunction partner layer and the junction partner layer iscircumferentially disposed on the absorber layer. In some embodiments,the absorber layer is copper-indium-gallium-diselenide and the junctionpartner layer is In₂Se₃, In₂S₃, ZnS, ZnSe, CdlnS, CdZnS, ZnIn₂Se₄,Zn_(1-x)Mg_(x)O, CdS, SnO₂, ZnO, ZrO₂, or doped ZnO.

In some embodiments, the cylindrical shaped solar cell further comprisesan intrinsic layer circumferentially disposed on the semiconductorjunction. In such embodiments, the transparent conductive layer isdisposed on the intrinsic layer rather than the semiconductor junction.

In some embodiments, the solar cell unit further comprises a fillerlayer circumferentially disposed on the transparent conductive layer. Insuch embodiments, the transparent tubular casing is circumferentiallydisposed on the filler layer thereby circumferentially sealing thetubular shaped solar cell.

In some embodiments, a water resistant layer is circumferentiallydisposed on the transparent conductive layer. In such embodiments, thetransparent tubular casing is circumferentially disposed on the waterresistant layer thereby circumferentially sealing the cylindrical shapedsolar cell.

In some embodiments, a water resistant layer is circumferentiallydisposed on the transparent conductive layer and a filler layercircumferentially is disposed on the water resistant layer. In suchembodiments the transparent tubular casing is circumferentially disposedon the filler layer thereby circumferentially sealing the cylindricalshaped solar cell.

In some embodiments, the solar cell unit further comprises a fillerlayer circumferentially disposed on the transparent conductive layer anda water resistant layer circumferentially disposed on the waterresistant layer. In such embodiments, the transparent tubular casing iscircumferentially disposed on the water resistant layer therebycircumferentially sealing the cylindrical shaped solar cell. In someembodiments, the solar cell unit further comprises an antireflectivecoating circumferentially disposed on the transparent tubular casing.

In some embodiments the cylindrically shaped solar cell furthercomprises at least one electrode strip, where each electrode strip inthe at least one electrode strip is overlayed on the transparentconductive layer of the solar cell along the long cylindrical axis ofthe solar cell. In some embodiments, the at least one electrode stripcomprises a plurality of electrode strips that are positioned at spacedintervals on the transparent conductive layer such that the plurality ofelectrode strips run parallel or approximately parallel to each otheralong the cylindrical axis of the solar cell. The plurality of electrodestrips can be spaced out at, for example, sixty degree intervals on asurface of the transparent conductive layer of the solar cell. In fact,the electrode strips in the plurality of electrode strips can be spacedout at any type of even interval or uneven interval on the surface ofthe transparent conductive layer of the solar cell. In some embodiments,a length of the cylindrical shaped solar cell is between 0.3 meters and2 meters. In some embodiments, an outer surface of the transparenttubular casing is textured.

Another aspect of the present invention provides a solar cell assemblycomprising a plurality of solar cell units. Each solar cell unit in theplurality of solar cell units has the structure of any of the solar cellunits described above. The solar cell units in the plurality of solarcell units are arranged in coplanar rows to form the solar cellassembly. In some embodiments, the solar assembly further comprises analbedo surface positioned to reflect sunlight into the plurality ofsolar cell units. In some embodiments, the albedo surface has an albedothat exceeds 95%. In some embodiments, the albedo surface is aLambertian, diffuse, or involute reflector surface. In some embodiments,a first solar cell unit and a second solar cell unit in the plurality ofsolar cell units is electrically arranged in series or in parallel.

Still another aspect of the present invention comprises a solar cellassembly comprising a plurality of solar cell units and a plurality ofinternal reflectors. Each of the solar cell units in the plurality ofsolar cell units has the structure of any of the solar cell unitsdescribed above. In this embodiment, the plurality of internalreflectors are arranged in coplanar rows in which internal reflectors inthe plurality of solar cell units abut solar cell units in the pluralityof solar cell units thereby forming the solar cell assembly. In someembodiments, an internal reflector in the plurality of internalreflectors has a hollow core. In some embodiments, an internal reflectorin the plurality of internal reflectors comprises a plastic casing witha layer of reflective material deposited on the plastic casing. In someembodiments, an internal reflector in the plurality of internalreflectors is a single piece made out of a reflective material. In someembodiments, a cross-sectional shape of an internal reflector in theplurality of internal reflectors is astroid. In some embodiments, across-sectional shape of an internal reflector in the plurality ofinternal reflectors is four-sided and a side of the four-sidedcross-sectional shape is linear, parabolic, concave, circular orelliptical. In some embodiments, a cross-sectional shape of an internalreflector in the plurality of internal reflectors is four-sided and aside of the four-sided cross-sectional shape defines a diffuse surfaceon the internal reflector.

Still another aspect of the invention provides a solar cell unitcomprising a cylindrical shaped solar cell, a filler layer, and atransparent tubular casing. In some embodiments the cylindrical shapedsolar cell comprises a cylindrical substrate, a back-electrodecircumferentially disposed on the cylindrical substrate, a semiconductorjunction circumferentially disposed on the back-electrode, and atransparent conductive layer disposed on the semiconductor junction. Thecylindrical substrate can be a hollowed cylinder (e.g., tube) or a solidcylinder. The filler layer is circumferentially disposed on thetransparent conductive layer and the transparent tubular casing iscircumferentially disposed onto the filler layer. In some embodiments inaccordance with this aspect of the invention, the semiconductor junctioncomprises an absorber layer and a junction partner layer and thejunction partner layer is circumferentially disposed on the absorberlayer while the absorber layer is circumferentially disposed on theback-electrode. In some embodiments in accordance with this aspect ofthe invention, the solar cell unit further comprises an antireflectivecoating circumferentially disposed on the transparent tubular casing.

Yet another aspect of the invention comprises a solar cell unitcomprising a cylindrical shaped solar cell, a water resistant layer, afiller layer, and a transparent tubular casing. The cylindrical shapedsolar cell comprises a cylindrical substrate, a back-electrodecircumferentially disposed on the cylindrical substrate, a semiconductorjunction circumferentially disposed on the back-electrode, and atransparent conductive layer disposed on the semiconductor junction. Thecylindrical substrate can be a solid cylinder or a hollowed cylinder(e.g., a tube). The water resistant layer is circumferentially disposedon the transparent conductive layer. The filler layer circumferentiallydisposed on the water resistant layer. The transparent tubular casing iscircumferentially disposed onto is filler layer.

Still another aspect of the invention provides a solar cell unitcomprising a cylindrical shaped solar cell, a filler layer, a waterresistant layer, and a transparent tubular casing. The cylindricalshaped solar cell comprises a cylindrical substrate, a back-electrodecircumferentially disposed on the cylindrical substrate, a semiconductorjunction circumferentially disposed on the back-electrode, and atransparent conductive layer disposed on the semiconductor junction. Thecylindrical substrate can be solid or hollowed (e.g., a tube). Thefiller layer is circumferentially disposed on the transparent conductivelayer. The water resistant layer is circumferentially disposed on thefiller layer. The transparent tubular casing is circumferentiallydisposed onto the water resistant layer.

4. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates interconnected solar cells in accordance with theprior art.

FIG. 2A illustrates a spherical solar cell including a p-type innerlayer and an n-type outer layer in accordance with the prior art.

FIG. 2B illustrates a tubular photovoltaic element comprising acylindrical silicon tube of n-type conductivity that has been subjectedto diffusion of boron into its outer surface to form an outerp-conductivity type region and thus a tubular solar cell in accordancewith the prior art.

FIG. 2C is a cross-sectional view of an elongated solar cell inaccordance with the prior art.

FIG. 2D is a cross-sectional view of a solar cell assembly in which aplurality of elongated solar cells are affixed to an electricallyconductive substrate in accordance with the prior art.

FIG. 2E is a cross-sectional view of a solar cell assembly disposed adistance away from a reflecting wall in accordance with the prior art.

FIG. 3A illustrates a photovoltaic element with tubular casing, inaccordance with an embodiment of the present invention.

FIG. 3B illustrates a cross-sectional view of an elongated solar cell ina transparent tubular casing, in accordance with an embodiment of thepresent invention.

FIG. 3C illustrates the multi-layer components of an elongated solarcell in accordance with an embodiment of the present invention.

FIG. 3D illustrates a transparent tubular casing, in accordance with anembodiment of the present invention.

FIG. 4A is a cross-sectional view of elongated solar cells in tubularcasing that are electrically arranged in series and geometricallyarranged in a parallel or near parallel manner, in accordance with anembodiment of the present invention.

FIG. 4B is a cross-sectional view taken about line 4B-4B of FIG. 4Adepicting the serial electrical arrangement of solar cells in anassembly, in accordance with an embodiment of the present invention.

FIG. 4C is a blow-up perspective view of region 4C of FIG. 4B,illustrating various layers in elongated solar cells, in accordance withone embodiment of the present invention.

FIG. 4D is a cross-sectional view of an elongated solar cell taken aboutline 4D-4D of FIG. 4B, in accordance with an embodiment of the presentinvention.

FIGS. 5A-5D illustrate semiconductor junctions that are used in variouselongated solar cells in various embodiments of the present invention.

FIG. 6A illustrates an extrusion blow molding method, in accordance withthe prior art.

FIG. 6B illustrates an injection blow molding method, in accordance withthe prior art.

FIG. 6C illustrates a stretch blow molding method, in accordance withthe prior art.

FIG. 7A is a cross-sectional view of elongated solar cells electricallyarranged in series in an assembly where counter-electrodes abutindividual solar cells, in accordance with another embodiment of thepresent invention.

FIG. 7B is a cross-sectional view taken about line 7B-7B of FIG. 7A thatdepicts the serial arrangement of the cylindrical solar cells in anassembly, in accordance with an embodiment of the present invention.

FIG. 7C is a perspective view an array of alternating tubular casings,in accordance with an embodiment of the present invention.

FIG. 8 is a cross-sectional view of elongated solar cells electricallyarranged in series in an assembly where counter-electrodes abutindividual solar cells and the outer TCO is cut, in accordance withanother embodiment of the present invention.

FIG. 9 is a cross-sectional view of elongated solar cells electricallyarranged in series in an assembly in which the inner metal electrode ishollowed, in accordance with an embodiment of the present invention.

FIG. 10 is a cross-sectional view of elongated solar cells electricallyarranged in series in an assembly in which a groove pierces thecounter-electrodes, transparent conducting oxide layer, and junctionlayers of the solar cells, in accordance with an embodiment of thepresent invention.

FIG. 11 illustrates a static concentrator for use in some embodiments ofthe present invention.

FIG. 12 illustrates a static concentrator used in some embodiments ofthe present invention.

FIG. 13 illustrates a cross-sectional view of a solar cell in accordancewith an embodiment of the present invention.

FIG. 14 illustrate molded tubular casing in accordance with someembodiments of the present invention.

FIG. 15 illustrates a perspective view of an elongated solar cellarchitecture with protruding electrode attachments, in accordance withan embodiment of the present invention.

FIG. 16 illustrates a perspective view of a solar cell architecture inaccordance with an embodiment of the present invention.

FIG. 17A illustrates light reflection on a specular surface, inaccordance with the prior art.

FIG. 17B illustrates light reflection on a diffuse surface, inaccordance with the prior art.

FIG. 17C illustrates light reflection on a Lambertian surface, inaccordance with the prior art.

FIG. 18A illustrates a circle and an involute of the circle, inaccordance with the prior art

FIG. 18B illustrates a cross-sectional view of a solar cell architecturein accordance with an embodiment of the present invention.

FIG. 19 illustrates a cross-sectional view of an array of alternatingtubular casings and internal reflectors, in accordance with anembodiment of the present invention.

FIG. 20A illustrates a suction loading assembly method in accordancewith the present invention.

FIG. 20B illustrates a pressure loading assembly method in accordancewith the present invention.

FIG. 20C illustrates a pour-and-slide loading assembly method inaccordance with the present invention.

FIG. 21 illustrates a partial cross-sectional view of an elongated solarcell in a transparent tubular casing, in accordance with an embodimentof the present invention.

FIG. 22 illustrates Q-type silicone, silsequioxane, D-type silicon, andM-type silicon, in accordance with the prior art.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings. Dimensions are not drawn to scale.

5. DETAILED DESCRIPTION

Disclosed herein are solar cell assemblies for converting solar energyinto electrical energy and more particularly to improved solar cells andsolar cell arrays. The solar cells of the present invention have anelongated cylindrical shape.

5.1 Basic Structure

The present invention provides individually circumferentially coveredcylindrical solar cell units 300 that are illustrated in perspectiveview in FIG. 3A and cross-sectional view in FIG. 3B. In a solar cellunit 300, an elongated cylindrical solar cell 402 (FIG. 3C) iscircumferentially covered by a transparent tubular casing 310 (FIG. 3D).In some embodiments, solar cell unit 300 comprises a solar cell 402coated with a transparent tubular casing 310. In some embodiments, onlyone end of elongated solar cell 402 is exposed by transparent tubularcasing 310 in order to form an electrical connection with adjacent solarcells 402 or other circuitry. In some embodiments, both ends ofelongated solar cell 402 are exposed by transparent tubular casing 310in order to form an electrical connection with adjacent solar cells 402or other circuitry. As used herein, the term cylindrical means objectshaving a cylindrical or approximately cylindrical shape. In fact,cylindrical objects can have irregular shapes so long as the object,taken as a whole, is roughly cylindrical. Such cylindrical shapes can besolid (e.g., a rod) or hollowed (e.g., a tube). As used herein, the termtubular means objects having a tubular or approximately tubular shape.In fact, tubular objects can have irregular shapes so long as theobject, taken as a whole, is roughly tubular.

Although most discussions in the present application pertaining to solarcell units 300 are in the context of either the encapsulated embodimentsor circumferentially covered embodiments, it is to be appreciated thatsuch discussions serve as no limitation to the scope of the presentinvention. Any transparent tubular casing that provides support andprotection to elongated solar cells and permits electrical connectionsbetween the elongated solar cells are within the scope of the systemsand methods of the present invention.

Descriptions of exemplary solar cells 402 are provided in this sectionas well as Sections 5.2 through 5.8. For instance, examples ofsemiconductor junctions that can be used in solar cells 402 arediscussed in Section 5.2. Exemplary systems and methods formanufacturing transparent tubular casing 310 are described in Section5.1.2. Systems and methods for coating solar cells 402 with transparenttubular casing 310 in order to form solar cell units 300 are found inSection 5.1.3. Solar cell units 300 can be assembled into solar cellassemblies of various sizes and shapes to generate electricity andpotentially heat water or other fluids.

FIG. 3B illustrates the cross-sectional view of an exemplary embodimentof solar cell unit 300. Other exemplary embodiments of solar cells(e.g., 402 in FIG. 4A) are also suitable for coating by transparenttubular casing 310.

Substrate 403. Substrate 403 serves as a substrate for solar cell 402.In some embodiments, substrate 403 is made of a plastic, metal, metalalloy, or glass. Substrate 403 is cylindrical shaped. In someembodiments, substrate 403 has a hollow core, as illustrated in FIG. 3B.In some embodiments, substrate 403 has a solid core. In someembodiments, the shape of substrate 403 is only approximately that of acylindrical object, meaning that a cross-section taken at a right angleto the long axis of substrate 403 defines an ellipse rather than acircle. As the term is used herein, such approximately shaped objectsare still considered cylindrically shaped in the present invention. Insome embodiments, substrate 403 is made of a urethane polymer, anacrylic polymer, a fluoropolymer, polybenzamidazole, polyimide,polytetrafluoroethylene, polyetheretherketone, polyamide-imide,glass-based phenolic, polystyrene, cross-linked polystyrene, polyester,polycarbonate, polyethylene, polyethylene,acrylonitrile-butadiene-styrene, polytetrafluoro-ethylene,polymethacrylate, nylon 6,6, cellulose acetate butyrate, celluloseacetate, rigid vinyl, plasticized vinyl, or polypropylene. In someembodiments, substrate 403 is made of aluminosilicate glass,borosilicate glass (e.g., Pyrex, Duran, Simax, etc.), dichroic glass,germanium/semiconductor glass, glass ceramic, silicate/fused silicaglass, soda lime glass, quartz glass, chalcogenide/sulphide glass,fluoride glass, pyrex glass, a glass-based phenolic, cereated glass, orflint glass. In some embodiments, substrate 403 is a solid cylindricalshape. Such solid cylindrical substrates 403 can be made out of aplastic, glass, metal, or metal alloy.

Back-electrode 404. A back-electrode 404 is circumferentially disposedon substrate 403. Back-electrode 404 serves as the first electrode inthe assembly. In general, back-electrode 404 is made out of any materialsuch that it can support the photovoltaic current generated by solarcell unit 300 with negligible resistive losses. In some embodiments,back-electrode 404 is composed of any conductive material, such asaluminum, molybdenum, tungsten, vanadium, rhodium, niobium, chromium,tantalum, titanium, steel, nickel, platinum, silver, gold, an alloythereof, or any combination thereof. In some embodiments, back-electrode404 is composed of any conductive material, such as indium tin oxide,titanium nitride, tin oxide, fluorine doped tin oxide, doped zinc oxide,aluminum doped zinc oxide, gallium doped zinc oxide, boron dope zincoxide indium-zinc oxide, a metal-carbon black-filled oxide, agraphite-carbon black-filled oxide, a carbon black-carbon black-filledoxide, a superconductive carbon black-filled oxide, an epoxy, aconductive glass, or a conductive plastic. As defined herein, aconductive plastic is one that, through compounding techniques, containsconductive fillers which, in turn, impart their conductive properties tothe plastic. In some embodiments, the conductive plastics used in thepresent invention to form back-electrode 404 contain fillers that formsufficient conductive current-carrying paths through the plastic matrixto support the photovoltaic current generated by solar cell unit 300with negligible resistive losses. The plastic matrix of the conductiveplastic is typically insulating, but the composite produced exhibits theconductive properties of the filler.

Semiconductor junction 410. A semiconductor junction 410 is formedaround back-electrode 404. Semiconductor junction 410 is anyphotovoltaic homojunction, heterojunction, heteroface junction, buriedhomojunction, a p-i-n junction or a tandem junction having an absorberlayer that is a direct band-gap absorber (e.g., crystalline silicon) oran indirect band-gap absorber (e.g., amorphous silicon). Such junctionsare described in Chapter 1 of Bube, Photovoltaic Materials, 1998,Imperial College Press, London, as well as Lugue and Hegedus, 2003,Handbook of Photovoltaic Science and Engineering, John Wiley & Sons,Ltd., West Sussex, England, each of which is hereby incorporated byreference herein in its entirety. Details of exemplary types ofsemiconductors junctions 410 in accordance with the present inventionare disclosed in Section 5.2, below. In addition to the exemplaryjunctions disclosed in Section 5.2, below, junctions 410 can bemultijunctions in which light traverses into the core of junction 410through multiple junctions that, preferably, have successfully smallerband gaps. In some embodiments, semiconductor junction 410 include acopper-indium-gallium-diselenide (CIGS) absorber layer.

Optional intrinsic layer 415. Optionally, there is a thin intrinsiclayer (i-layer) 415 circumferentially coating semiconductor junction410. The i-layer 415 can be formed using any undoped transparent oxideincluding, but not limited to, zinc oxide, metal oxide, or anytransparent material that is highly insulating. In some embodiments,i-layer 415 is highly pure zinc oxide.

Transparent conductive layer 412. Transparent conductive layer 412 iscircumferentially disposed on the semiconductor junction layers 410thereby completing the circuit. As noted above, in some embodiments, athin i-layer 415 is circumferentially disposed on semiconductor junction410. In such embodiments, transparent conductive layer 412 iscircumferentially disposed on i-layer 415. In some embodiments,transparent conductive layer 412 is made of tin oxide SnO_(x) (with orwithout fluorine doping), indium-tin oxide (ITO), doped zinc oxide(e.g., aluminum doped zinc oxide, gallium doped zinc oxide, boron dopezinc oxide), indium-zinc oxide or any combination thereof. In someembodiments, transparent conductive layer 412 is either p-doped orn-doped. In some embodiments, transparent conductive layer is made ofcarbon nanotubes. Carbon nanotubes are commercially available, forexample from Eikos (Franklin, Mass.) and are described in U.S. Pat. No.6,988,925, which is hereby incorporated by reference herein in itsentirety. For example, in embodiments where the outer semiconductorlayer of junction 410 is p-doped, transparent conductive layer 412 canbe p-doped. Likewise, in embodiments where the outer semiconductor layerof junction 410 is n-doped, transparent conductive layer 412 can ben-doped. In general, transparent conductive layer 412 is preferably madeof a material that has very low resistance, suitable opticaltransmission properties (e.g., greater than 90%), and a depositiontemperature that will not damage underlying layers of semiconductorjunction 410 and/or optional i-layer 415. In some embodiments,transparent conductive layer 412 is an electrically conductive polymermaterial such as a conductive polytiophene, a conductive polyaniline, aconductive polypyrrole, a PSS-doped PEDOT (e.g., Bayrton), or aderivative of any of the foregoing. In some embodiments, transparentconductive layer 412 comprises more than one layer, including a firstlayer comprising tin oxide SnO_(x) (with or without fluorine doping),indium-tin oxide (ITO), indium-zinc oxide, doped zinc oxide (e.g.,aluminum doped zinc oxide, gallium doped zinc oxide, boron dope zincoxide) or a combination thereof and a second layer comprising aconductive polytiophene, a conductive polyaniline, a conductivepolypyrrole, a PSS-doped PEDOT (e.g., Bayrton), or a derivative of anyof the foregoing. Additional suitable materials that can be used to formtransparent conductive layer are disclosed in United States Patentpublication 2004/0187917A1 to Pichler, which is hereby incorporated byreference herein in its entirety.

Optional electrode strips 420. In some embodiments in accordance withthe present invention, counter-electrode strips or leads 420 aredisposed on transparent conductive layer 412 in order to facilitateelectrical current flow. In some embodiments, electrode strips 420 arethin strips of electrically conducting material that run lengthwisealong the long axis (cylindrical axis) of the cylindrically shaped solarcell, as depicted in FIG. 4A. In some embodiments, optional electrodestrips are positioned at spaced intervals on the surface of transparentconductive layer 412. For instance, FIG. 3B, electrode strips 420 runparallel to each other and are spaced out at ninety degree intervalsalong the cylindrical axis of the solar cell. In some embodiments,electrode strips 420 are spaced out at five degree, ten degree, fifteendegree, twenty degree, thirty degree, forty degree, fifty degree, sixtydegree, ninety degree or 180 degree intervals on the surface oftransparent conductive layer 412. In some embodiments, there is a singleelectrode strip 420 on the surface of transparent conductive layer 412.In some embodiments, there is no electrode strip 420 on the surface oftransparent conductive layer 412. In some embodiments, there is two,three, four, five, six, seven, eight, nine, ten, eleven, twelve, fifteenor more, or thirty or more electrode strips on transparent conductivelayer 412, all running parallel, or near parallel, to each down the long(cylindrical) axis of the solar cell. In some embodiments electrodestrips 420 are evenly spaced about the circumference of transparentconductive layer 412, for example, as depicted in FIG. 3B. Inalternative embodiments, electrode strips 420 are not evenly spacedabout the circumference of transparent conductive layer 412. In someembodiments, electrode strips 420 are only on one face of the solarcell. Elements 403, 404, 410, 415 (optional), and 412 of FIG. 3Bcollectively comprise solar cell 402 of FIG. 3A. In some embodiments,electrode strips 420 are made of conductive epoxy, conductive ink,copper or an alloy thereof, aluminum or an alloy thereof, nickel or analloy thereof, silver or an alloy thereof, gold or an alloy thereof, aconductive glue, or a conductive plastic.

In some embodiments, there are electrode strips that run along the long(cylindrical) axis of the solar cell and these electrode strips areinterconnected to each other by grid lines. These grid lines can bethicker than, thinner than, or the same width as the electrode strips.These grid lines can be made of the same or different electricallymaterial as the electrode strips.

In some embodiments, electrode strips 420 are deposited on transparentconductive layer 412 using ink jet printing. Examples of conductive inkthat can be used for such strips include, but are not limited to silverloaded or nickel loaded conductive ink. In some embodiments epoxies aswell as anisotropic conductive adhesives can be used to constructelectrode strips 420. In typical embodiments, such inks or epoxies arethermally cured in order to form electrode strips 420.

Optional filler layer 330. The addition of counter-electrode strips orleads 420 often renders the shape of the circular solar cells irregular.Care is taken to exclude air from the solar cell unit to avoidoxidation. Accordingly, in some embodiments of the present invention, asdepicted in FIG. 3B, a filler layer 330 of sealant such as ethyl vinylacetate (EVA), silicone, silicone gel, epoxy, polydimethyl siloxane(PDMS), RTV silicone rubber, polyvinyl butyral (PVB), thermoplasticpolyurethane (TPU), a polycarbonate, an acrylic, a fluoropolymer, and/ora urethane is coated over transparent conductive layer 412 to seal outair and, optionally, to provide complementary fitting to a transparenttubular casing 310. In some embodiments, filler layer 330 is a Q-typesilicone, a silsequioxane, a D-type silicon, or an M-type silicon.However, in some embodiments, optional filler layer 330 is not neededeven when one or more electrode strips 420 are present. Additionalsuitable materials for optional filler layer 330 are disclosed inSection 5.1.4, below.

Transparent tubular casing 310. Transparent tubular casing 310 iscircumferentially disposed on transparent conductive layer 412 and/oroptional filler layer 330. In some embodiments tubular casing 310 ismade of plastic or glass. In some embodiments, elongated solar cells402, after being properly modified for future packaging as describedbelow, are sealed in the transparent tubular casing 310. As shown inFIG. 4A, transparent tubular casing 310 fits over the outermost layer ofelongated solar cell 402. In some embodiments, elongated solar cell 402is inside transparent tubular casing 310 such that adjacent elongatedsolar cells 402 do not form electric contact with each other except atthe ends of the solar cells. Methods, such as heat shrinking, injectionmolding, or vacuum loading, can be used to construct transparent tubularcasing 310 such they exclude oxygen and water from the system as well asto provide complementary fitting to the underlying solar cell 402. Insome embodiments, transparent tubular casing 310, for example asdepicted in FIG. 14, can be used to cover elongated solar cells 402.

In some embodiments, transparent tubular casing 310 is made of aurethane polymer, an acrylic polymer, polymethylmethacrylate (PMMA), afluoropolymer, silicone, poly-dimethyl siloxane (PDMS), silicone gel,epoxy, ethyl vinyl acetate (EVA), perfluoroalkoxy fluorocarbon (PFA),nylon/polyamide, cross-linked polyethylene (PEX), polyolefin,polypropylene (PP), polyethylene terephtalate glycol (PETG),polytetrafluoroethylene (PTFE), thermoplastic copolymer (for example,ETFE®, which is a derived from the polymerization of ethylene andtetrafluoroethylene: TEFLON® monomers), polyurethane/urethane, polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), Tygon®, vinyl, Viton®,or any combination or variation thereof. Additional suitable materialsfor optional filler layer 330 are disclosed in Section 5.1.4, below.

In some embodiments, transparent tubular casing 310 comprises aplurality of transparent tubular casing layers. In some embodiments,each transparent tubular casing is composed of a different material. Forexample, in some embodiments, transparent tubular casing 310 comprises afirst transparent tubular casing layer and a second transparent tubularcasing layer. Depending on the exact configuration of the solar cell,the first transparent tubular casing layer is disposed on thetransparent conductive layer 412, optional filler layer 330 or the waterresistant layer. The second transparent tubular casing layer is disposedon the first transparent tubular casing layer.

In some embodiments, each transparent tubular casing layer has differentproperties. In one example, the outer transparent tubular casing layerhas excellent UV shielding properties whereas the inner transparenttubular casing layer has good water proofing characteristics. Moreover,the use of multiple transparent tubular casing layers can be used toreduce costs and/or improve the overall properties of transparenttubular casing 310. For example, one transparent tubular casing layermay be made of an expensive material that has a desired physicalproperty. By using one or more additional transparent tubular casinglayers, the thickness of the expensive transparent tubular casing layermay be reduced, thereby achieving a savings in material costs. Inanother example, one transparent tubular casing layer may have excellentoptical properties (e.g., index of refraction, etc.) but be very heavy.By using one or more additional transparent tubular casing layers, thethickness of the heavy transparent tubular casing layer may be reduced,thereby reducing the overall weight of transparent tubular casing 310.

Optional water resistant layer. In some embodiments, one or more layersof water resistant layer are coated over solar cell 402 to prevent thedamaging effects of water molecules. In some embodiments, this waterresistant layer is circumferentially coated onto transparent conductivelayer 412 prior to depositing optional filler layer 330 and encasing thesolar cell 402 in transparent tubular casing 310. In some embodiments,such water resistant layers are circumferentially coated onto optionalfiller layer 330 prior to encasing the solar cell 402 in transparenttubular casing 310. In some embodiments, such water resistant layers arecircumferentially coated onto transparent tubular casing 310 itself. Inembodiments where a water resistant layer is provided to seal molecularwater from solar cell 402, it is important that the optical propertiesof the water resistant layer not interfere with the absorption ofincident solar radiation by solar cell 402. In some embodiments, thiswater resistant layer is made of clear silicone, SiN, SiO_(x)N_(y),SiO_(x), or Al₂O₃, where x and y are integers. In some embodiments,water resistant layer is made of a Q-type silicone, a silsequioxane, aD-type silicon, or an M-type silicon.

Optional antireflective coating. In some embodiments, an optionalantireflective coating is also circumferentially disposed on transparenttubular casing 310 to maximize solar cell efficiency. In someembodiments, there is a both a water resistant layer and anantireflective coating deposited on transparent tubular casing 310. Insome embodiments, a single layer serves the dual purpose of a waterresistant layer and an anti-reflective coating. In some embodiments,antireflective coating, made of MgF₂, silicone nitrate, titaniumnitrate, silicon monoxide (SiO), or silicon oxide nitrite. In someembodiments, there is more than one layer of antireflective coating. Insome embodiments, there is more than one layer of antireflective coatingand each layer is made of the same material. In some embodiments, thereis more than one layer of antireflective coating and each layer is madeof a different material.

In some embodiments, some of the layers of multi-layered solar cells 402are constructed using cylindrical magnetron sputtering techniques. Insome embodiments, some of the layers of multi-layered solar cells 402are constructed using conventional sputtering methods or reactivesputtering methods on long tubes or strips. Sputtering coating methodsfor long tubes and strips are disclosed in for example, Hoshi et al.,1983, “Thin Film Coating Techniques on Wires and Inner Walls of SmallTubes via Cylindrical Magnetron Sputtering,” Electrical Engineering inJapan 103:73-80; Lincoln and Blickensderfer, 1980, “AdaptingConventional Sputtering Equipment for Coating Long Tubes and Strips,” J.Vac. Sci. Technol. 17:1252-1253; Harding, 1977, “Improvements in a dcReactive Sputtering System for Coating Tubes,” J. Vac. Sci. Technol.14:1313-1315; Pearce, 1970, “A Thick Film Vacuum Deposition System forMicrowave Tube Component Coating,” Conference Records of 1970 Conferenceon Electron Device Techniques 208-211; and Harding et al., 1979,“Production of Properties of Selective Surfaces Coated onto Glass Tubesby a Magnetron Sputtering System,” Proceedings of the InternationalSolar Energy Society 1912-1916, each of which is hereby incorporated byreference herein in its entirety.

Optional fluorescent material. In some embodiments, a fluorescentmaterial (e.g., luminescent material, phosphorescent material) is coatedon a surface of a layer of solar cell 300. In some embodiments, thefluorescent material is coated on the luminal surface and/or theexterior surface of transparent tubular casing 310. In some embodiments,the fluorescent material is coated on the outside surface of transparentconductive oxide 412. In some embodiments, solar cell 300 includes anoptional filler layer 300 and the fluorescent material is coated on theoptional filler layer. In some embodiments, solar cell 300 includes awater resistant layer and the fluorescent material is coated on thewater resistant layer. In some embodiments, more than one surface of asolar cell 300 is coated with optional fluorescent material. In someembodiments, the fluorescent material absorbs blue and/or ultravioletlight, which some semiconductor junctions 410 of the present inventiondo not use to convert to electricity, and the fluorescent material emitslight in visible and/or infrared light which is useful for electricalgeneration in some solar cells 300 of the present invention.

Fluorescent, luminescent, or phosphorescent materials can absorb lightin the blue or UV range and emit visible light. Phosphorescentmaterials, or phosphors, usually comprise a suitable host material andan activator material. The host materials are typically oxides,sulfides, selenides, halides or silicates of zinc, cadmium, manganese,aluminum, silicon, or various rare earth metals. The activators areadded to prolong the emission time.

In some embodiments, phosphorescent materials are incorporated in thesystems and methods of the present invention to enhance light absorptionby solar cell 300. In some embodiments, the phosphorescent material isdirectly added to the material used to make optional transparent tubularcasing 310. In some embodiments, the phosphorescent materials are mixedwith a binder for use as transparent paints to coat various outer orinner layers of solar cell 300, as described above.

Exemplary phosphors include, but are not limited to, copper-activatedzinc sulfide (ZnS:Cu) and silver-activated zinc sulfide (ZnS:Ag). Otherexemplary phosphorescent materials include, but are not limited to, zincsulfide and cadmium sulfide (ZnS:CdS), strontium aluminate activated byeuropium (SrAlO₃:Eu), strontium titanium activated by praseodymium andaluminum (SrTiO3:Pr, Al), calcium sulfide with strontium sulfide withbismuth ((Ca,Sr)S:Bi), copper and magnesium activated zinc sulfide(ZnS:Cu,Mg), or any combination thereof.

Methods for creating phosphor materials are known in the art. Forexample, methods of making ZnS:Cu or other related phosphorescentmaterials are described in U.S. Pat. Nos. 2,807,587 to Butler et al.;3,031,415 to Morrison et al.; 3,031,416 to Morrison et al.; 3,152,995 toStrock; 3,154,712 to Payne; 3,222,214 to Lagos et al.; 3,657,142 toPoss; 4,859,361 to Reilly et al., and 5,269,966 to Karam et al., each ofwhich is hereby incorporated by reference herein in its entirety.Methods for making ZnS:Ag or related phosphorescent materials aredescribed in U.S. Pat. Nos. 6,200,497 to Park et al., 6,025,675 to Iharaet al.; 4,804,882 to Takahara et al., and 4,512,912 to Matsuda et al.,each of which is hereby incorporated herein by reference in itsentirety. Generally, the persistence of the phosphor increases as thewavelength decreases. In some embodiments, quantum dots of CdSe orsimilar phosphorescent material can be used to get the same effects. SeeDabbousi et al., 1995, “Electroluminescence from CdSequantum-dot/polymer composites,” Applied Physics Letters 66 (11):1316-1318; Dabbousi et al., 1997 “(CdSe)ZnS Core-Shell Quantum Dots:Synthesis and Characterization of a Size Series of Highly LuminescentNanocrystallites,” J. Phys. Chem. B, 101: 9463-9475; Ebenstein et al.,2002, “Fluorescence quantum yield of CdSe:ZnS nanocrystals investigatedby correlated atomic-force and single-particle fluorescence microscopy,”Applied Physics Letters 80: 4033-4035; and Peng et al., 2000, “Shapecontrol of CdSe nanocrystals,” Nature 404: 59-61; each of which ishereby incorporated by reference herein in its entirety.

In some embodiments, optical brighteners are used in the optionalfluorescent layers of the present invention. Optical brighteners (alsoknown as optical brightening agents, fluorescent brightening agents orfluorescent whitening agents) are dyes that absorb light in theultraviolet and violet region of the electromagnetic spectrum, andre-emit light in the blue region. Such compounds include stilbenes(e.g., trans-1,2-diphenylethylene or (E)-1,2-diphenylethene). Anotherexemplary optical brightener that can be used in the optionalfluorescent layers of the present invention is umbelliferone(7-hydroxycoumarin), which also absorbs energy in the UV portion of thespectrum. This energy is then re-emitted in the blue portion of thevisible spectrum. More information on optical brighteners is in Dean,1963, Naturally Occurring Oxygen Ring Compounds, Butterworths, London;Joule and Mills, 2000, Heterocyclic Chemistry, 4^(th) edition, BlackwellScience, Oxford, United Kingdom; and Barton, 1999, Comprehensive NaturalProducts Chemistry 2: 677, Nakanishi and Meth-Cohn eds., Elsevier,Oxford, United Kingdom, 1999.

Circumferentially disposed. In the present invention, layers of materialare successively circumferentially disposed on a cylindrical substrate403 in order to form a solar cell. As used herein, the termcircumferentially disposed is not intended to imply that each such layerof material is necessarily deposited on an underlying layer. In fact,the present invention teaches methods by which such layers are molded orotherwise formed on an underlying layer. Nevertheless, the termcircumferentially disposed means that an overlying layer is disposed onan underlying layer such that there is no annular space between theoverlying layer and the underlying layer. Furthermore, as used herein,the term circumferentially disposed means that an overlying layer isdisposed on at least fifty percent of the perimeter of the underlyinglayer. Furthermore, as used herein, the term circumferentially disposedmeans that an overlying layer is disposed along at least half of thelength of the underlying layer.

Circumferentially sealed. In the present invention, the termcircumferentially sealed is not intended to imply that an overlyinglayer or structure is necessarily deposited on an underlying layer orstructure. In fact, the present invention teaches methods by which suchlayers or structures (e.g., transparent tubular casing 310) are moldedor otherwise formed on an underlying layer or structure. Nevertheless,the term circumferentially sealed means that an overlying layer orstructure is disposed on an underlying layer or structure such thatthere is no annular space between the overlying layer or structure andthe underlying layer or structure. Furthermore, as used herein, the termcircumferentially sealed means that an overlying layer is disposed onthe full perimeter of the underlying layer. In typical embodiments, alayer or structure circumferentially seals an underlying layer orstructure when it is circumferentially disposed around the fullperimeter of the underlying layer or structure and along the full lengthof the underlying layer or structure. However, the present inventioncontemplates embodiments in which a circumferentially sealing layer orstructure does not extend along the full length of an underlying layeror structure.

5.1.1 Solar Cell Unit Assemblies

FIG. 4A illustrates a cross-sectional view of the arrangement of threesolar cell units 300 arranged in a coplanar fashion in order to form asolar cell assembly 400. FIG. 4B provides a cross-sectional view withrespect to line 4B-4B of FIG. 4A. In FIG. 4, back-electrode 404 isdepicted as a solid cylindrical substrate. However, in some embodimentsin accordance with FIG. 4, rather than being a solid cylindricalsubstrate, back-electrode is a thin layer of electrically conductingmaterial circumferentially disposed on substrate 403 as depicted in FIG.3B. All other layers in FIG. 4 are as illustrated in FIG. 3B. Like inFIG. 3B, optional filler layer 330 in the embodiments depicted in FIG. 4is optional.

As can be seen with FIGS. 4A and 4B, each elongated cell 402 has alength that is great compared to the diameter d of its cross-section. Anadvantage of the architecture shown in FIG. 4A is that there is no frontside contact that shades solar cells 402. Such a front side contact isfound in known devices (e.g., elements 10 of FIG. 2B). Another advantageof the architecture shown in FIG. 4A is that elongated cells 402 areelectrically connected in series rather than in parallel. In such aseries configuration, the voltage of each elongated cell 402 is summed.This serves to increase the voltage across the system, thereby keepingthe current down, relative to comparable parallel architectures, andminimizes resistive losses. A serial electrical arrangement ismaintained by arranging all or a portion of the elongated solar cells402 as illustrated in FIGS. 4A and 4B. Another advantage of thearchitecture shown in FIG. 4A is that the resistance loss across thesystem is low. This is because each electrode component of the circuitis made of highly conductive material. For example, as noted above,conductive core 404 of each solar cell 402 is made of a conductivematerial such as metal. In the alternative, where conductive core 404 isnot a solid, but rather comprises a back-electrode layercircumferentially deposited on substrate 403, the back-electrode layer404 is highly conductive. Regardless of whether back-electrode 404 is ina solid configuration as depicted in FIG. 4 or a thin layer as depictedin FIG. 3B, such back-electrodes 404 carry current without anappreciable current loss due to resistance. While larger conductivecores 404 (FIG. 4) and/or thicker back-electrode 404 (FIG. 3B) ensurelow resistance, transparent conductive layers encompassing such largerconductive cores 404 must carry current further to contacts(counter-electrode strips or leads) 420. Thus, there is an upper boundon the size of conductive cores 404 and/or substrate 403. In view ofthese and other considerations, diameter d is between 0.5 millimeters(mm) and 20 mm in some embodiments of the present invention. Thus,conductive core 404 (FIG. 4) and/or substrate 403 (FIG. 3B) are sized sothat they are large enough to carry a current without appreciableresistive losses, yet small enough to allow transparent conductive 412to efficiently deliver current to counter-electrode strip 420.

The advantageous low resistance nature of the architecture illustratedin FIG. 4A is also facilitated by the highly conductive properties ofcounter-electrode strip 420. In some embodiments, for example,counter-electrode strips 420 are composed of a conductive epoxy (e.g.,silver epoxy) or conductive ink and the like. For example, in someembodiments, counter-electrode strips 420 are formed by depositing athin metallic layer on a suitable substrate and then patterning thelayer into a series of parallel strips. Each counter-electrode strip 420is affixed to a solar cell 402 with a conductive epoxy along the lengthof a solar cell 402, as shown in FIG. 4D. In some embodiments,counter-electrode strips 420 are formed directly on solar cells 402. Inother embodiments, counter-electrode strips 420 are formed on the outertransparent conductive layer 412, as illustrated in FIG. 3B. In someembodiments, connections between counter-electrode strip 420 toelectrodes 433 are established in series as depicted in FIG. 4B.

Still another advantage of the architecture illustrated in FIG. 4A isthat the path length through the absorber layer (e.g., layer 502, 510,520, or 540 of FIG. 5) of semiconductor junction 410 is, on average,longer than the path length through of the same type of absorber layerhaving the same width but in a planar configuration. Thus, the elongatedarchitecture illustrated in FIG. 4A allows for the design of thinnerabsorption layers relative to analogous planar solar cell counterparts.In the elongated architecture, the thinner absorption layer absorbs thelight because of the increased path length through the layer. Becausethe absorption layer is thinner relative to comparable planar solarcells, there is less resistance and, hence, an overall increase inefficiency in the cell relative to analogous planar solar cells.Additional advantages of having a thinner absorption layer that stillabsorbs sufficient amounts of light is that such absorption layersrequire less material and are thus cheaper. Furthermore, thinnerabsorption layers are faster to make, thereby further loweringproduction costs.

Another advantage of elongated solar cells 402 illustrated in FIG. 4A isthat they have a relatively small surface area, relative to comparableplanar solar cells, and they possess radial symmetry. Each of theseproperties allow for the controlled deposition of doped semiconductorlayers necessary to form semiconductor junction 410. The smaller surfacearea, relative to conventional flat panel solar cells, means that it iseasier to present a uniform vapor across the surface during depositionof the layers that form semiconductor junction 410. The radial symmetrycan be exploited during the manufacture of the cells in order to ensureuniform composition (e.g., uniform material composition, uniform dopantconcentration, etc.) and/or uniform thickness of individual layers ofsemiconductor junction 410. For example, the conductive core 404 uponwhich layers are deposited to make solar cells 402 can be rotated alongits longitudinal axis during such deposition in order to ensure uniformmaterial composition and/or uniform thickness.

The cross-sectional shape of solar cells 402 is generally circular inFIG. 4B. In other embodiments, solar cell 402 bodies with aquadrilateral cross-section or an elliptical shaped cross-section andthe like are used. In fact, there is no limit on the cross-sectionalshape of solar cells 402 in the present invention, so long as the solarcells 402 maintain a general overall rod-like shape in which theirlength is much larger than their diameter and they possess some form ofcross-sectional radial symmetry or approximate cross-sectional radialsymmetry.

In some embodiment, as illustrated in FIG. 4A, a first and secondelongated solar cell (rod-shaped solar cell) 402 are electricallyconnected in series by an electrical contact 433 that connects theback-electrode 404 (first electrode) of the first elongated solar cell402 to the corresponding counter-electrode strip 420 of the secondelongated solar cell 402. Thus, as illustrated in FIG. 4A, elongatedsolar cells 402 are the basic unit that respectively forms thesemiconductor layer 410, the transparent conductive layer 412, and themetal conductive core 404 of the elongated solar cell 402. In someembodiments, the elongated solar cells 402 are multiply arranged in arow parallel or nearly parallel with respect to each other and rest uponindependent leads (counter-electrodes) 420 that are electricallyisolated from each other. Advantageously, in the configurationillustrated in FIG. 4A, elongated solar cells 402 can receive directlight through transparent tubular casing 310.

In some embodiments, not all elongated solar cells 402 in assembly 400are electrically arranged in series. For example, in some embodiments,there are pairs of elongated solar cells 402 that are electricallyarranged in parallel. A first and second elongated solar cell can beelectrically connected in parallel, and are thereby paired, by using afirst electrical contact (e.g., an electrically conducting wire, etc.,not shown) that joins the conductive core 404 of a first elongated solarcell to the second elongated solar cell. To complete the parallelcircuit, transparent conductive 412 of the first elongated solar cell402 is electrically connected to transparent conductive layer 412 of thesecond elongated solar cell 402 either by contacting the transparentconductive layers of the two elongated solar cells either directly orthrough a second electrical contact (not shown). The pairs of elongatedsolar cells are then electrically arranged in series. In someembodiments, three, four, five, six, seven, eight, nine, ten, eleven ormore elongated solar cells 402 are electrically arranged in parallel.These parallel groups of elongated solar cells 402 are then electricallyarranged in series.

FIG. 4C is an enlargement of region 4C of FIG. 4B in which a portion ofback-electrode 404 and transparent conductive layer 412 have been cutaway to illustrate the positional relationship between counter-electrodestrip 420, electrode 433, back-electrode 404, semiconductor layer 410,and transparent conductive layer 412. Furthermore, FIG. 4C illustrateshow electrical contact 433 joins back-electrode 404 of one elongatedsolar cell 402 to counter-electrode 420 of another solar cell 402.

One advantage of the configuration illustrated in FIG. 4 is thatelectrical contacts 433 that serially connect solar cells 402 togetheronly need to be placed on one end of assembly 400, as illustrated inFIG. 4B. However, encapsulation shields each solar cell 402 fromunwanted electrical contacts from adjacent solar cells 402, makingencapsulation relatively simple. Thus, referring to FIG. 4D, which is across-sectional view of an elongated solar 402 cell taken about line4D-4D of FIG. 4B, it is possible to completely seal far-end 455 of solarcell 402 with transparent tubular casing 310 in the manner illustrated.In some embodiments, the layers in this seal are identical to the layerscircumferentially disposed lengthwise on conductive core 404, namely, inorder of deposition on conductive core 404 and/or substrate 403,semiconductor junction 410, optional thin intrinsic layer (i-layer) 415,and transparent conductive layer 412. In such embodiments, end 455 canreceive sunlight and therefore contribute to the electrical generatingproperties of the solar cell 402. In some embodiments, transparenttubular casing 310 open at both ends of solar cell 402 such thatelectrical contacts can be extended from either end of the solar cell.

FIG. 4D also illustrates how, in some embodiments, the various layersdeposited on conductive core 404 are tapered at end 466 where electricalcontacts 433 are found. For instance, a terminal portion ofback-electrode 404 is exposed, as illustrated in FIG. 4D. In otherwords, semiconductor junction 410, optional i-layer 415, and transparentconductive layer 412 are stripped away from a terminal portion ofconductive core 404. Furthermore, a terminal portion of semiconductorjunction 410 is exposed as illustrated in FIG. 4D. That is, optionali-layer 415 and transparent conductive layer 412 are stripped away froma terminal portion of semiconductor junction 410. The remaining portionsof the conductive core 404, semiconductor junction 410, optional i-layer415, and transparent conductive layer 412 are coated by transparenttubular casing 310. Such a configuration is advantageous because itprevents a short from developing between transparent conductive layer412 and conductive core 404. In FIG. 4D, elongated solar cell 402 ispositioned on counter-electrode strip 420 which, in turn, is positionedagainst electrically resistant transparent tubular casing 310. However,there is no requirement that counter-electrode strip 420 make contactwith electrically resistant transparent tubular casing 310. In fact, insome embodiments, elongated solar cells 402 and their correspondingcounter-electrode strips 420 are sealed within transparent conductivelayer 412 such that there is no unfavorable electrical contact. In suchembodiments, elongated solar cells 402 and corresponding electrodestrips 420 are fixedly held in place by a sealant such as ethyl vinylacetate or silicone. In some embodiments in accordance with the presentinvention, counter-electrode strips 420 are replaced with metal wiresthat are attached to the sides of solar cell 402. In some embodiments inaccordance with the present invention, solar cells 402 implement asegmented design to eliminate the requirement of additional wire- orstrip-like counter-electrodes. Details on segmented solar cell designare found in copending U.S. patent application Ser. No. to bedetermined, attorney docket number 11653-007-999, entitled “MonolithicIntegration of Cylindrical Solar Cells,” filed Mar. 18, 2006, which ishereby incorporated by reference herein in its entirety.

FIG. 4D further provides a perspective view of electrical contacts 433that serially connect elongated solar cells 402. For instance, a firstelectrical contact 433-1 electrically interfaces with counter-electrode420 whereas a second electrical contact 433-2 electrically interfaceswith back-electrode 404 (the first electrode of elongated solar cell402). First electrical contact 433-1 serially connects thecounter-electrode of elongated solar cell 402 to the back-electrode 404of another elongated solar cell. Second electrical contact 433-2serially connects the back-electrode 404 of elongated solar cell 402 tothe counter-electrode 420 of another elongated solar cell 402, as shownin FIG. 4B. Such an electrical configuration is possible regardless ofwhether back-electrode 404 is itself a solid cylindrical substrate or isa layer of electrically conducting material on a substrate 403 asdepicted in FIG. 3B. Each solar cell 402 is coated by a transparenttubular casing 310.

In addition, FIG. 4D provides an encapsulated solar cell 402 where anoptional filler layer 330 and a transparent tubular casing 310 cover thesolar cell, leaving only one end 466 to establish electrical contracts.It is to be appreciated that, in some embodiments, the optional fillerlayer 330 and transparent tubular casing 310 are configured such thatboth ends (e.g., 455 and 466 in FIG. 4D) of the elongated solar cell 402are available to establish electrical contacts.

FIG. 7A illustrates a solar cell assembly 700 in accordance with anotherembodiment of the present invention. Solar cell assembly 700 comprises aplurality of elongated solar cells 402, each encapsulated in transparenttubular casing 310. Each elongated solar cell 402 in the plurality ofelongated solar cells has a back-electrode 404 configured as a firstelectrode. In the embodiments depicted in FIG. 7A, back electrode 404 isa solid cylindrical electrically conducting substrates. However, inalternative embodiments in accordance with FIG. 7, back-electrode 404 isa thin film of electrically conducting material deposited on a hollowedtubular shaped substrate as in the case of FIG. 3B. The principalstaught in FIG. 7 apply to each such form of back-electrode 404. In FIG.7, a semiconductor junction 410 is circumferentially disposed on theconductive core 402 and a transparent conductive layer 412 iscircumferentially disposed on semiconductor junction 410. In someembodiments, the plurality of elongated solar cells 402 aregeometrically arranged in a parallel or a near parallel manner therebyforming a planar array having a first face (facing side 733 of assembly700) and a second face (facing side 766 of assembly 700). The pluralityof elongated solar cells is arranged such that one or more elongatedsolar cells in the plurality of elongated solar cells do not contactadjacent elongated solar cells. In some embodiments, the plurality ofelongated solar cells is arranged such that each of the elongated solarcells in the plurality of elongated solar cells does not directlycontact (through transparent conductive layer 412) adjacent elongatedsolar cells 402. In some embodiments, the plurality of elongated solarcells is arranged such that each of the elongated solar cells in theplurality of elongated solar cells does directly contact the outertransparent tubular casing 310 of adjacent elongated solar cells 402.

In some embodiments, there is a first groove 777-1 and a second groove777-2 that each runs lengthwise on opposing sides of solar cell 402. InFIG. 7A, some but not all grooves 777 are labeled. In some embodiments,there is a counter-electrode 420 in one or both grooves of the solarcells. In the embodiment illustrated in FIG. 6A, there is acounter-electrode fitted lengthwise in both the first and second groovesof each solar cell in the plurality of solar cells. Such a configurationis advantageous because it reduces the path length of current drawn offof transparent conductive layer 412. In other words, the maximum lengththat current must travel in transparent conductive layer 412 before itreaches a counter-electrode 420 is a quarter of the circumference of thetransparent conductive layer. By contrast, in configurations where thereis only a single counter-electrode 420 associated with a given solarcell 402, the maximum length that current must travel in transparentconductive layer 412 before it reaches a counter-electrode 420 is a fullhalf of the circumference of the transparent conductive layer 412. Thepresent invention encompasses grooves 777 that have a broad range ofdepths and shape characteristics and is by no means limited to the shapeof the grooves 777 illustrated in FIG. 7A. In general, any groove shape777 that runs along the long axis of a solar cell 402 and that canaccommodate all or part of counter-electrode 420 is within the scope ofthe present invention. For example, in some embodiments not illustratedby FIG. 7A, each groove 777 is patterned so that there is a tight fitbetween the contours of the groove 777 and the counter-electrode 420.

As illustrated in FIG. 7A, there are a plurality of metalcounter-electrodes 420, and each respective elongated solar cell 402 inthe plurality of elongated solar cells is bound to at least a firstcorresponding metal counter-electrode 420 in the plurality of metalcounter-electrodes such that the first metal counter-electrode lies in agroove 777 that runs lengthwise along the respective elongated solarcell. Furthermore, in the solar cell assembly illustrated in FIG. 7A,each respective elongated solar cell 402 is bound to a secondcorresponding metal counter-electrode 420 such that the second metalcounter-electrode lies in a second groove 777 that runs lengthwise alongthe respective elongated solar cell 402. As further illustrated in FIG.7A, the first groove 777 and the second groove 777 are on opposite orsubstantially opposite sides of the respective elongated solar cell 402and run along the long axis of the cell.

In some embodiments, transparent tubular casing 310, such as thetransparent tubular casing 310 depicted in FIG. 14, is used to encaseelongated solar cells 402. Because it is important to exclude air fromthe solar cell unit 402, an optional filler layer 330 iscircumferentially disposed between solar cell 402 and transparenttubular casing 310 in the manner illustrated in FIG. 7A in someembodiments of the present invention. In some embodiments, filler layer330 prevents the seepage of oxygen and water into solar cells 402. Insome embodiments, filler layer 330 comprises EVA or silicone. In someembodiments, the individually encased solar cells 402 are assembled intoa planar array as depicted in FIG. 7A. The plurality of elongated solarcells 402 are configured to receive direct light from both face 733 andface 766 of the planar array.

FIG. 7B provides a cross-sectional view with respect to line 7B-7B ofFIG. 7A. Solar cells 402 are electrically connected to other in seriesby arranging the solar cells such that they do not touch each other, asillustrated in FIGS. 7A and 7B and by the use of electrical contacts asdescribed below in conjunction with FIG. 7B. Although the individualsolar cells are shown separate from each other to reveal the encasing bytransparent tubular casing 310, no actual separation distance betweensolar cells 402 is required since transparent tubular casing 310 shieldsthe individual solar cells 402 of solar cell unit 300 from anyunfavorable electrical contacts. However, tight space or no spacepacking is not a required for individually shielded solar cell unit 300.In fact, the presence of the transparent tubular casing 310 providesmore versatility in the solar cell assembly. For instance, in someembodiments, the distance between adjacent solar cell units 300 is 0microns or greater, 0.1 microns or greater, 0.5 microns or greater, orbetween 1 and 5 microns, or optimally correlated with the size anddimensions of the solar cell units 300.

Referring to FIG. 7B, serial electrical contact between solar cells 402is made by electrical contacts 788 that electrically connect theback-electrode 404 of one elongated solar cell 402 to the correspondingcounter-electrodes 120 of a different solar cell 402. FIG. 7B furtherillustrates a cutaway of metal conductive core 404 and semiconductorjunction 410 in one solar cell 402 to further illustrate thearchitecture of solar cells 402.

The solar cell assembly illustrated in FIG. 7 has several advantages.First, the planar arrangement of the solar cells 402 leaves almost zeropercent shading in the assembly. For instance, the assembly can receivedirect sunlight from both face 733 and face 766. Second, in embodimentswhere individually encapsulated solar cells 402 are aligned parallel toeach other with no or little space separation, the structure iscompletely self-supporting. Still another advantage of the assembly isease of manufacture. Unlike solar cells such as that depicted in FIG.2B, no complicated grid or transparent conductive oxide on glass isrequired. For example, to assemble a solar cell 402 and itscorresponding counter-electrodes 420 together to complete the circuitillustrated in FIG. 7A, counter-electrode 420, when it is in the form ofa wire, can be covered with conductive epoxy and dropped in the groove777 of solar cell 402 and allowed to cure.

As illustrated in FIG. 7B, conductive core 404, junction 410, andtransparent conductive layer 412 are flush with each other at end 789 ofelongated solar cells 402. In contrast, at end 799 conductive coreprotrudes a bit with respect to junction 410 and transparent conductivelayer 412 as illustrated. Junction 410 also protrudes a bit at end 799with respect to transparent conductive layer 412. The protrusion ofconductive core 404 at end 799 means that the sides of a terminalportion of the conductive core 404 are exposed (e.g., not covered byjunction 410 and transparent conductive layer 412). The purpose of thisconfiguration is to reduce the chances of shorting counter-electrode 420(or the epoxy used to mount the counter-electrode in groove 777) withtransparent conductive layer 412. In some embodiments, all or a portionof the exposed surface area of counter-electrodes 420 are shielded withan electrically insulating material in order to reduce the chances ofelectrical shortening. For example, in some embodiments, the exposedsurface area of counter-electrodes 420 in the boxed regions of FIG. 7Bis shielded with an electrically insulating material.

Still another advantage of the assembly illustrated in FIG. 7 is thatthe counter-electrode 420 can have much higher conductivity withoutshadowing. In other words, counter-electrode 420 can have a substantialcross-sectional size (e.g., 1 mm in diameter when solar cell 402 has a 6mm diameter). Thus, counter-electrode 420 can carry a significant amountof current so that the wires can be as long as possible, thus enablingthe fabrication of larger panels.

The series connections between solar cells 402 can be between pairs ofsolar cells 402 in the manner depicted in FIG. 7B. However, theinvention is not so limited. In some embodiments, two or more solarcells 402 are grouped together (e.g., electrically connected in aparallel fashion) to form a group of solar cells and then such groups ofsolar cells are serially connected to each other. Therefore, the serialconnections between solar cells can be between groups of solar cellswhere such groups have any number of solar cells 402 (e.g., 2, 3, 4, 5,6, etc.). However, FIG. 7B illustrates a preferred embodiment in whicheach contact 788 serially connects only a pair of solar cells 402.

Yet another advantage of the assembly illustrated in FIG. 7B is thattransparent tubular casing 310 is circumferentially disposed on solarcells 402. In some embodiments, an optional filler layer 330 liesbetween the outer surface of solar cell 402 and the inner surface oftransparent tubular casing 310. Although FIG. 7B only depicts electricalcircuitry at one end of adjacent solar cell units 300, it is possiblefor electrical circuitry to be established at both ends of solar cellunits 300 or between the two ends of solar cell units 300.

The solar cell design in accordance with the present invention isadvantageous in that each individual solar cell 402 is encapsulated bytransparent tubular casing 310. transparent tubular casing 310 is atleast partial transparent and made of non-conductive material such asplastics or glass. Accordingly, solar cell assemblies made according tothe present design do not require insulator lengthwise between eachsolar cell 402. Yet another embodiment of solar cell assembly 700 isthat there is no extra absorption loss from a transparent conductivelayer or a metal grid on one side of the assembly. Further, assembly 700has the same performance or absorber area exposed on both sides 733 and766. This makes assembly 700 symmetrical.

Still another advantage of assembly 700 is that all electrical contacts788 end at the same level (e.g., in the plane of line 7B-7B of FIG. 7A).As such, they are easier to connect and weld with very little substratearea wasted at the end. This simplifies construction of the solar cells402 while at the same time serves to increase the overall efficiency ofsolar cell assembly 700. This increase in efficiency arises because thewelds can be smaller.

Although not illustrated in FIG. 7, in some embodiments in accordancewith FIG. 7, there is an intrinsic layer 415 circumferentially disposedbetween the semiconductor junction 410 and the transparent conductivelayer 412 in an elongated solar cell 402 in the plurality of elongatedsolar cells 402. Intrinsic layer 415 can be made of an undopedtransparent oxide such as zinc oxide, metal oxide, or any transparentmetal that is highly insulating. In some embodiments, the semiconductorjunction 410 of solar cells 402 in assembly 700 comprise an innercoaxial layer and an outer coaxial layer where the outer coaxial layercomprises a first conductivity type and the inner coaxial layercomprises a second, opposite, conductivity type. In an exemplaryembodiment, the inner coaxial layer comprisescopper-indium-gallium-diselenide (CIGS) whereas the outer coaxial layercomprises In₂Se₃, In₂S₃, ZnS, ZnSe, CdlnS, CdZnS, ZnIn₂Se₄,Zn_(1-x)Mg_(x)O, CdS, SnO₂, ZnO, ZrO₂, or doped ZnO. In some embodimentsnot illustrated by FIG. 7, conductive cores 404 in solar cells 402 arehollowed.

FIG. 8 illustrates a solar cell assembly 800 of the present inventionthat is identical to solar cell assembly 700 of the present inventionwith the exception that transparent conductive layer 412 is interruptedby breaks 810 that run along the long axis of solar cells 402 and cutcompletely through transparent conductive layer 412. In the embodimentillustrated in FIG. 8, there are two breaks 810 that run the length ofsolar cell 402. The effect of such breaks 810 is that they electricallyisolate the two counter-electrodes 420 associated with each solar cell402 in solar cell assembly 800. There are many ways in which breaks 800can be made. For example, a laser or an HCl etch can be used.

In some embodiments, not all elongated solar cells 402 in assembly 800are electrically arranged in series. For example, in some embodiments,there are pairs of elongated solar cells 402 that are electricallyarranged in parallel. A first and second elongated solar cell can beelectrically connected in parallel, and are thereby paired, by using afirst electrical contact (e.g., an electrically conducting wire, etc.,not shown) that joins the conductive core 404 of a first elongated solarcell to the second elongated solar cell. To complete the parallelcircuit, transparent conductive layer 412 of the first elongated solarcell 402 is electrically connected to transparent conductive layer 412of the second elongated solar cell 402 either by contacting thetransparent conductive layers of the two elongated solar cells eitherdirectly or through a second electrical contact (not shown). The pairsof elongated solar cells are then electrically arranged in series. Insome embodiments, three, four, five, six, seven, eight, nine, ten,eleven or more elongated solar cells 402 are electrically arranged inparallel. These parallel groups of elongated solar cells 402 are thenelectrically arranged in series.

In some embodiments, transparent tubular casing 310, such as depicted inFIG. 14, is used to encase elongated solar cells 402. Because it isimportant to exclude air from the solar cell unit 402, a filler layer330 may be used to prevent oxidation of the solar cell 402. Asillustrated in FIG. 8, filler layer 330 (for example EVA) preventsseepage of oxygen and water into solar cells 402. Filler layer isdisposed between solar cell 402 and the inner layer of transparenttubular casing 310. In some embodiments, the individually encapsulatedsolar cells 402 are assembled into a planar array as depicted in FIG. 8.

FIG. 9 illustrates a solar cell assembly 900 of the present invention inwhich back-electrodes 404 are hollowed. In fact, back-electrode 404 canbe hollowed in any of the embodiments of the present invention. Oneadvantage a hollowed back-electrode 404 design is that it reduces theoverall weight of the solar cell assembly. Back-electrode 404 ishollowed when there is a channel that extends lengthwise through all ora portion of back-electrode 404. In some embodiments, back-electrode 404is metal tubing. In some embodiments, back-electrode 404 is a thin layerof electrically conducting material, e.g. molybdenum, that is depositedon a substrate 403 as illustrated in FIG. 3B.

In some embodiments, not all elongated solar cells 402 in assembly 900are electrically arranged in series. For example, in some embodiments,there are pairs of elongated solar cells 402 that are electricallyarranged in parallel. The pairs of elongated solar cells are thenelectrically arranged in series. In some embodiments, three, four, five,six, seven, eight, nine, ten, eleven or more elongated solar cells 402are electrically arranged in parallel. These parallel groups ofelongated solar cells 402 are then electrically arranged in series.

In some embodiments, a transparent tubular casing 310, for example asdepicted in FIG. 14, can be used to circumferentially cover elongatedsolar cells 402. Because it is important to exclude air from the solarcell unit 402, additional sealant may be used to prevent oxidation ofthe solar cell 402. Alternatively, as illustrated in FIG. 9, an optionalfiller layer 330 (for example, EVA or silicone, etc.) may be used toprevent seepage of oxygen and water into solar cells 402. In someembodiments, the individually encased solar cells 402 are assembled intoa planar array as depicted in FIG. 9. FIG. 10 illustrates a solar cellassembly 1000 of the present invention in which counter-electrodes 420,transparent conductive layers 412, and junctions 410 are pierced, in themanner illustrated, in order to form two discrete junctions in parallel.In some embodiments, transparent tubular casing 310, for example asdepicted in FIG. 14, may be used to encase elongated solar cells 402with or without optional filler layer 330.

FIG. 15 illustrates an elongated solar cell 402 in accordance with thepresent invention. A transparent tubular casing 310 encases theelongated solar cell 402, leaving only ends of electrodes 420 exposed toestablish suitable electrical connections. The ends of the elongatedsolar cell 402 are stripped and conductive layer 404 is exposed. As inprevious embodiments, back-electrode 404 serves as the first electrodein the assembly and transparent conductive layer 412 on the exteriorsurface of each elongated solar cell 402 serves as thecounter-electrode. In some embodiments in accordance with the presentinvention as illustrated in FIG. 15, however, protrudingcounter-electrodes 420 and electrodes 440, which are attached to theelongated solar cell 402, provide convenient electrical connection.

In typical embodiments as shown in FIG. 15, there is a first groove677-1 and a second groove 677-2 that each runs lengthwise on opposingsides of elongated solar cell 402. In some embodiments,counter-electrodes 420 are fitted into grooves 677 in the mannerillustrated in FIG. 15. Typically, such counter-electrodes 420 are gluedinto grooves 677 using a conductive ink or conductive glue. For example,CuPro-Cote (available from Lessemf.com, Albany, N.Y.), which is asprayable metallic coating system using a non-oxidizing copper as aconductor, can be used. In preferred embodiments, counter-electrodes 420are fitted in to grooves 677 and then a bead of conductive ink orconductive glue is applied. As in previous embodiments, the presentinvention encompasses grooves 677 that have a broad range of depths andshape characteristics and is by no means limited to the shape of thegrooves 677 illustrated in FIG. 15. In general, any type of groove 677that runs along the long axis of a first solar cell 402 and that canaccommodate all or part of counter-electrode 420 is within the scope ofthe present invention. Counter-electrodes 420 conduct current from thecombined layer 410/(415)/412. At the regions at both ends of elongatedsolar cell 402, counter-electrodes 420 are sheathed as shown in FIG. 15so that they are electrically isolated from conductive layer 404. Theends of protruding counter-electrodes 420, however, are unsheathed sothey can form electric contact with additional devices.

In the embodiments as depicted in FIG. 15, a second set of electrodes440 are attached to the exposed back-electrode 404. The second set ofelectrodes 440 conduct current from back-electrode 404. As illustratedin FIG. 15, an embodiment in accordance with the present invention hastwo electrodes 440 attached at two opposing ends of each elongated solarcell 402. Typically, electrodes 440 are glued onto back-electrode 404using a conductive ink or conductive glue. For example, CuPro-Cote canbe used. In some embodiments, electrodes 440 are glued to layer 404 andthen a bead of conductive ink or conductive glue is applied. Care istaken so that electrodes 440 are not in electrical contact with layer410/(415)/412. Additionally, electrodes 440 in the present inventionhave a broad range of lengths and widths and shape characteristics andare by no means limited to the shape of 440 illustrated in FIG. 15. Inthe embodiments as shown in FIG. 15, the two electrodes 440 on oppositeends of the elongated solar cell 402 are not on the same side of thesolar cell cylinder. The first electrode 440 is on the bottom side ofthe elongated solar cell 402 while the second electrode 440 is on thetop side of the elongated solar cell 402. Such an arrangementfacilitates the connection of the solar cells in a serial manner. Insome embodiments in accordance with the present invention, the twoelectrodes 440 can be on the same side of elongated solar cell 402.

In some embodiments, each electrode 440 is made of a thin strip ofconductive material that is attached to conductive layer 404/1304 (FIG.15). In some embodiments, each electrode 440 is made of a conductiveribbon of metal (e.g., copper, aluminum, gold, silver, molybdenum, or analloy thereof) or a conductive ink. As will be explained in conjunctionwith subsequent drawings, counter-electrode 420 and electrodes 440 areused to electrically connect elongated solar cells 402, preferably inseries.

5.1.2 Transparent Tubular Casing

A transparent tubular casing 310, as depicted in FIGS. 3A through 3C,seals a solar cell unit 402 to provide support and protection to thesolar cell. The size and dimensions of transparent tubular casing 310are determined by the size and dimension of individual solar cells 402in a solar cell assembly unit 402. Transparent tubular casing 310 may bemade of glass, plastic or any other suitable material. Examples ofmaterials that can be used to make transparent tubular casing 310include, but are not limited to, glass (e.g., soda lime glass), acrylicssuch as polymethylmethacrylate, polycarbonate, fluoropolymer (e.g.,Tefzel or Teflon), polyethylene terephthalate (PET), Tedlar, or someother suitable transparent material.

5.1.2.1 Transparent Tubular Casing Construction

In some embodiments, transparent tubular casing 310 is constructed usingblow molding. Blow molding involves clamping the ends of a softened tubeof polymers, which can be either extruded or reheated, inflating thepolymer against the mold walls with a blow pin, and cooling the productby conduction or evaporation of volatile fluids in the container. Threegeneral types of blow molding are extrusion blow molding, injection blowmolding, and stretch blow molding. Extrusion blow molding is used tomake items of weight greater than twelve ounces. Injection blow moldingachieves accurate wall thickness. Stretch blow molding is typically usedfor difficult to blow crystalline and crystallizable polymers such aspolypropylene and polyethylene terephthalate. U.S. Pat. No. 237,168describes a process for blow molding (e.g., 602 in FIG. 6A). Other formsof blow molding include low density polyethylene (LDPE) blow molding,high density polyethylene (HDPE) blow molding and polypropylene (PP)blow molding

Extrusion blow molding. As depicted in FIG. 6A, the extrusion blowmolding method comprises a Parison (e.g., 602 in FIG. 6A) and moldhalves that close onto the Parison (e.g., 604 in FIG. 6A). In extrusionblow molding (EBM), material is melted and extruded into a hollow tube(e.g., a Parison as depicted in FIG. 6A). The Parison is then capturedby closing it into a cooled metal mold. Air is then blown into theParison, inflating it into the shape of the hollow bottle, container orpart. After the material has cooled sufficiently, the mold is opened andthe part is ejected.

EBM processes consist of either continuous or intermittent extrusion ofthe Parison 602. The types of EBM equipment may be categorizedaccordingly. Typical continuous extrusion equipments usually compriserotary wheel blow molding systems and a shuttle machinery thattransports the finished products from the Parison. Exemplaryintermittent extrusion machinery comprises a reciprocating screwmachinery and an accumulator head machinery. Basic polymers, such as PP,HDPE, PVC and PET are increasingly being coextruded with high barrierresins, such as EVOH or Nylon, to provide permeation resistance towater, oxygen, CO₂ or other substances.

Compared to injection molding, blow molding is a low pressure process,with typical blow air pressures of 25 to 150 psi. This low pressureprocess allows the production of economical low-force clamping stations,while parts can still be produced with surface finishes ranging fromhigh gloss to textured. The resulting low stresses in the molded partsalso help make the containers resistant to strain and environmentalstress cracking.

Injection blow molding. In injection blow molding (IBM), as depicted inFIG. 6B, material is injection molded onto a core pin (e.g., 612 in FIG.6B); then the core pin is rotated to a blow molding station (e.g., 614in FIG. 6B) to be inflated and cooled. The process is divided in tothree steps: injection, blowing and ejection. A typical IBM machine isbased on an extruder barrel and screw assembly which melts the polymer.The molten polymer is fed into a manifold where it is injected throughnozzles into a hollow, heated preform mold (e.g., 614 in FIG. 6B). Thepreform mold forms the external shape and is clamped around a mandrel(the core rod, e.g., 612 in FIG. 6B) which forms the internal shape ofthe preform. The preform consists of a fully formed bottle/jar neck witha thick tube of polymer attached, which will form the body.

The preform mold opens and the core rod is rotated and clamped into thehollow, chilled blow mold. The core rod 612 opens and allows compressedair into the preform 614, which inflates it to the finished articleshape. After a cooling period the blow mold opens and the core rod isrotated to the ejection position. The finished article is stripped offthe core rod and leak-tested prior to packing. The preform and blow moldcan have many cavities, typically three to sixteen depending on thearticle size and the required output. There are three sets of core rods,which allow concurrent preform injection, blow molding and ejection.

Stretch blow molding In the stretch blow molding (SBM) process, asdepicted in FIG. 6C, the material is first molded into a “preform,”e.g., 628 in FIG. 6C, using the injection molded process. A typical SBMsystem comprises a stretch blow pin (e.g., 622 in FIG. 6C), an airentrance (e.g., 624 in FIG. 6C), mold vents (e.g., 626 in FIG. 6C), apreform (e.g., 628 in FIG. 6C), and cooling channels (e.g., 632 in FIG.6C). These preforms are produced with the necks of the bottles,including threads (the “finish”) on one end. These preforms arepackaged, and fed later, after cooling, into an EBM blow moldingmachine. In the SBM process, the preforms are heated, typically usinginfrared heaters, above their glass transition temperature, then blownusing high pressure air into bottles using metal blow molds. Usually thepreform is stretched with a core rod as part of the process (e.g., as inposition 630 in FIG. 6C). The stretching of some polymers, such as PET(polyethylene terepthalate), results in strain hardening of the resinand thus allows the bottles to resist deforming under the pressuresformed by carbonated beverages, which typically approach 60 psi.

FIG. 6C shows what happens inside the blow mold. The preform is firststretched mechanically with a stretch rod. As the rod travels downlow-pressure air of 5 to 25 bar (70 to 350 psi) is introduced blowing a‘bubble’. Once the stretch rod is fully extended, high-pressure air ofup to 40 bar (580 psi) blows the expanded bubble into the shape of theblow mold.

Plastic tube manufacturing. In some embodiments, transparent tubularcasing 310 is made of plastic rather than glass. Production oftransparent tubular casing 310 in such embodiments differs from glasstransparent tubular casing 310 production even though the basic moldingmechanisms remain the same. A typical plastic transparent tubular casing310 manufacturing process comprises the following steps: extrusion,heading, decorating, and capping, with the latter two steps beingoptional.

In some embodiments, transparent tubular casing 310 is made usingextrusion molding. A mixture of resin is placed into an extruder hopper.The extruder is temperature controlled as the resin is fed through toensure proper melt of the resin. The material is extruded through a setof sizing dies that are encapsulated within a right angle cross sectionattached to the extruder. The forming die controls the shape oftransparent tubular casing 310. The formed plastic sleeve cools underblown air or in a water bath and hardens on a moving belt. After coolingstep, the formed plastic sleeve is ready for cutting to a given lengthby a rotating knife.

The forming die controls the shape of the transparent tubular casing310. In some embodiments in accordance with the present invention, asdepicted in FIG. 14, the forming dies are custom-made such that theshape of transparent tubular casing 310 complements the shape of thesolar cell unit 402. The forming die also controls the wall thickness ofthe transparent tubular casing 310. In some embodiments in accordancewith the present invention, transparent tubular casing 310 has a wallthickness of 2 mm or thicker, 1 mm or thicker, 0.5 mm or thicker, 0.3 mmor thicker, or of any thickness between 0 and 0.3 mm.

During the production of one open-ended transparent tubular casing 310,the balance of the manufacturing process can be accomplished in one ofthree ways. The most common method in the United States is the “downs”process of compression, molding the head onto the tube. In this process,the sleeve is placed on a conveyor that takes it to the headingoperation where the shoulder of the head is bound to the body of thetube while, at the same time, the thread is formed. The sleeve is thenplaced on a mandrel and transferred down to the slug pick-up station.The hot melt strip or slug is fused onto the end of the sleeve and thentransferred onto the mold station. At this point, in one operation, theangle of the shoulder, the thread and the orifice are molded at the endof the sleeve. The head is then cooled, removed from the mold, andtransferred into a pin conveyor. Two other heading methods are used inthe United States and are found extensively worldwide: injection moldingof the head to the sleeve, and an additional compression molding methodwhereby a molten donut of resin material is dropped into the moldstation instead of the hot melt strip or slug. Transparent tubularcasing 310 with one open-end are suitable to encase solar cellembodiments as depicted in FIGS. 3, 4, 7, 8, 9, 10 or 11. Plastic tubingwith both ends open may be used to encase solar cell embodiments asdepicted in FIGS. 3 and 15.

The headed transparent tubular casing 310 is then conveyed to theaccumulator. The accumulator is designed to balance the heading anddecorating operation. From here, the transparent tubular casing 310 maygo to the decorating operation. Inks for the press are premixed andplaced in the fountains. At this point, the ink is transferred onto aplate by a series of rollers. The plate then comes in contact with arubber blanket, picking up the ink and transferring it onto thecircumference of the transparent tubular casing 310. The wet ink on thetube is cured by ultra-violet light or heat. In the embodiments inaccordance with the present invention, transparency is required in thetube products so the color process is unnecessary. However, a similarmethod may be used to apply a protective coating to transparent tubularcasing 310.

After decorating, a conveyor transfers the tube to the capping stationwhere the cap is applied and torqued to the customer's specifications.The capping step is unnecessary for the scope of this invention.

Additional glass fabrication methods. Glass is a preferred materialchoice for transparent tubular casing 310 relative to plastics becauseglass provides a complete seal against molecular water from solar cell402 and therefore provides protection and helps to maintain theperformance and prolong the lifetime of solar cell 402. Similar toplastics, glass may be made into transparent tubular casing 310 usingthe standard blow molding technologies. In addition, techniques such ascasting, extrusion, drawing, pressing, heat shrinking or otherfabrication processes may also be applied to manufacture suitable glasstransparent tubular casing 310 to circumferentially cover and/orencapsulate solar cells 402. Molding technologies, in particularmicromolding technologies for microfabrication, are discussed in greaterdetail in Madou, Fundamentals of Microfabrication, Chapter 6, pp.325-379, second edition, CRC Press, New York, 2002; Polymer EngineeringPrinciples: Properties, Processes, and Tests for Design, HanserPublishers, New York, 1993; and Lee, Understanding Blow Molding, firstedition., Hanser Gardner Publications, Munich, Cincinnati, 2000, each ofwhich is hereby incorporated herein by reference in its entirety.

5.1.2.2 Exemplary Materials for Transparent Tubular Casing

Transparent tubular casing made of glass. In some embodiments,transparent tubular casing 310 is made of glass. In its pure form, glassis a transparent, relatively strong, hard-wearing, essentially inert,and biologically inactive material that can be formed with very smoothand impervious surfaces. The present invention contemplates a widevariety of glasses for transparent tubular casing 310, some of which aredescribed in this section and others of which are know to those of skillin the relevant arts. Common glass contains about 70% amorphous silicondioxide (SiO₂), which is the same chemical compound found in quartz, andits polycrystalline form, sand. Common glass is used in some embodimentsof the present invention to make transparent tubular casing 310.However, common glass is brittle and will break into sharp shards. Thus,in some embodiments, the properties of common glass are modified, oreven changed entirely, with the addition of other compounds or heattreatment.

Pure silica (SiO₂) has a melting point of about 2000° C., and can bemade into glass for special applications (for example, fused quartz).Two other substances are always added to common glass to simplifyprocessing. One is soda (sodium carbonate Na₂CO₃), or potash, theequivalent potassium compound, which lowers the melting point to about1000° C. However, the soda makes the glass water-soluble, which isundesirable, so lime (calcium oxide, CaO) is the third component, addedto restore insolubility. The resulting glass contains about 70% silicaand is called a soda-lime glass. Soda-lime glass is used in someembodiments of the present invention to make transparent tubular casing310.

Besides soda-lime, most common glass has other ingredients added tochange its properties. Lead glass, such as lead crystal or flint glass,is more ‘brilliant’ because the increased refractive index causesnoticeably more “sparkles”, while boron may be added to change thethermal and electrical properties, as in Pyrex. Adding barium alsoincreases the refractive index. Thorium oxide gives glass a highrefractive index and low dispersion, and was formerly used in producinghigh-quality lenses, but due to its radioactivity has been replaced bylanthanum oxide in modern glasses. Large amounts of iron are used inglass that absorbs infrared energy, such as heat absorbing filters formovie projectors, while cerium(IV) oxide can be used for glass thatabsorbs UV wavelengths (biologically damaging ionizing radiation). Glasshaving on or more of any of these additives is used in some embodimentsof the present invention to make transparent tubular casing 310.

Common examples of glass material include but are not limited toaluminosilicate, borosilicate (e.g., Pyrex, Duran, Simax), dichroic,germanium/semiconductor, glass ceramic, silicate/fused silica, sodalime, quartz, chalcogenide/sulphide, cereated glass, and fluoride glassand transparent tubular casing 310 can be made of any of thesematerials.

In some embodiments, transparent tubular casing 310 is made of soda limeglass. Soda lime glass is softer than borosilicate and quartz, makingscribe cutting easier and faster. Soda Lime glass is very low cost andeasy to mass produce. However, Soda lime glass has poor thermal shockresistance. Thus, soda lime glass is best used for transparent tubularcasing 310 in thermal environments where heating is very uniform andgradual. As a result, when solar cells 402 are encased by transparenttubular casing 310 made from soda lime glass, such cells are best usedin environments where temperature does not drastically fluctuate.

In some embodiments, transparent tubular casing 310 are made of glassmaterial such as borosilicate glass. Trade names for borosilicate glassinclude but are not limited to Pyrex® (Corning), Duran® (Schott Glass),and Simax® (Kavalier). Like most glasses, the dominant component ofborosilicate glass is SiO₂ with boron and various other elements added.Borosilicate glass is easier to hot work than materials such as quartz,making fabrication less costly. Material cost for borosilicate glass isalso considerably less than fused quartz. Compared to most glass, exceptfused quartz, borosilicate glass has low coefficient of expansion, threetimes less than soda lime glass. This makes borosilicate glass useful inthermal environments, without the risk of breakage due to thermal shock.Like soda lime glass, a float process can be used to make relatively lowcost optical quality sheet borosilicate glass in a variety of thicknessfrom less than 1 mm to over 30 mm thick. Relative to quartz,borosilicate glass is easily moldable. In addition, borosilicate glasshas minimum devitrification when molding and flame working. This meanshigh quality surfaces can be maintained when molding and slumping.Borosilicate glass is thermally stable up to 500° C. for continuous use.Borosilicate glass is also more resistant to non-fluorinated chemicalsthan household soda lime glass and mechanically stronger and harder thansoda lime glass. Borosilicate is usually two to three times moreexpensive than soda lime glass.

Soda lime and borosilicate glass are only given as examples toillustrate the various aspects of consideration when using glassmaterial to fabricate transparent tubular casing 310. The precedingdiscussion imposes no limitation to the scope of the invention. Indeed,transparent tubular casing 310 can be made with glass such as, forexample, aluminosilicate, borosilicate (e.g., Pyrax®, Duran®, Simax®),dichroic, germanium/semiconductor, glass ceramic, silicate/fused silica,soda lime, quartz, chalcogenide/sulphide, cereated glass and/or fluorideglass.

Transparent tubular casing made of plastic. In some embodiments,transparent tubular casing 310 is made of clear plastic. Plastics are acheaper alternative to glass. However, plastic material is in generalless stable under heat, has less favorable optical properties and doesnot prevent molecular water from penetrating through transparent tubularcasing 310. The last factor, if not rectified, damages solar cells 402and severely reduces their lifetime. Accordingly, in some embodiments,the water resistant layer described in Section 5.1.1. is used to preventwater seepage into the solar cells 402 when transparent tubular casing310 is made of plastic.

A wide variety of materials can be used in the production of transparenttubular casing 310, including, but not limited to, ethyl vinyl acetate(EVA), perfluoroalkoxy fluorocarbon (PFA), nylon/polyamide, cross-linkedpolyethylene (PEX), polyolefin, polypropylene (PP), polyethyleneterephtalate glycol (PETG), polytetrafluoroethylene (PTFE),thermoplastic copolymer (for example, ETFE®, which is a derived from thepolymerization of ethylene and tetrafluoroethylene: TEFLON® monomers),polyurethane/urethane, polyvinyl chloride (PVC), polyvinylidene fluoride(PVDF), Tygon®, Vinyl, and Viton®.

5.1.2.3 Available Commercial Sources of Transparent Tubing Products

There are ample commercial sources for obtaining or custom manufacturingtransparent tubular casing 310. Technologies for manufacturing plasticor glass tubing have been standardized and customized plastic or glasstubing are commercially available from numerous companies. A search onGlobalSpec database for “clear round plastic or glass tubing,” a webcenter of engineering resources (www.globalspec.com; GlobalSpec Inc.Troy, N.Y.), results in over 950 catalog products. Over 180 companiesmake specialty pipe, tubing, hose and fittings. For example, ClippardInstrument Laboratory, Inc. (Cincinnati, Ohio) provides Nylon, Urethaneor Plastic Polyurethane tubing that is as thin as 0.4 mm. Coast Wire &Plastic Tech., Inc. (Carson, Calif.) manufactures a comprehensive lineof polyvinylidene fluoride clear round plastic tubing product under thetrademark SUMIMARK™. Their product has a wall thickness as thin as 0.3mm. Parker Hannifin/Fluid Connectors/Parflex Division (Ravenna, Ohio)provides vinyl, plastic polyurethane, polyether base, or polyurethanebased clear plastic tubing of 0.8 mm or 1 mm thickness. Similarpolyurethane products may also be found in Pneumadyne, Inc (Plymouth,Minn.). Saint-Gobain High-Performance Materials (U.S.A) further providesa line of 30 Tygon® tubing products of 0.8 mm in thickness. VindumEngineering, Inc. (San Ramon, Calif.) also provides clear PFA Teflontube of 0.8 mm in thickness. NewAge Industries, Inc. (Southampton, Pa.)provides 63 clear round plastic tubing products that have a wallthickness of 1 mm or thinner. In particular, VisiPak Extrusion (Arnold,Mo.), a division of Sinclair & Rush, Inc., provides clear round plastictubing product as thin as 0.5 mm. Cleartec Packaging (St. Louis, Mo., adivision of MOCAP Inc.) manufactures clear round plastic tubing as thinas 0.3 mm.

In addition, numerous companies can manufacture clear round plastic orglass tubing with customized specification such as even thinner wall.Some examples are Elasto Proxy Inc. (Boisbriand, Canada), FlexEnterprises, Inc. (Victor, N.Y.), Grob, Inc. (Grafton, Wis.), MercerGasket & Shim (Bellmawr, N.J.), New England Small Tube Corporation(Litchfield, N.H.), Precision Extrusion, Inc. (Glens Falls, N.Y.), andPSI Urethanes, Inc. (Austin, Tex.).

5.1.3 Integrating Solar Cells into Transparent Tubular Casings

In the present invention, gaps or spaces between transparent tubularcasing 310 and solar cell 402 are eliminated in order to avoid adverseeffects such as oxidation. Thus, in the present invention, there is noannular space between the inside wall of transparent tubular casing 310and the outer wall of solar cell 402. In some embodiments (e.g., FIG.3B), a filler layer 330 is provided to seal a solar cell unit 402 fromadverse exposure to water or oxygen. In some embodiments, wheretransparent tubular casing 310 is made of glass and therefore exposureto water is no longer a concern, a filler layer 330 may be eliminatedsuch that solar cells 402 directly contacts transparent tubular casing310.

In some embodiments, custom-designed transparent tubular casing 310,made of either glass or plastics or other suitable transparent material,may be used to encase the corresponding embodiments of solar cell 402 toachieve tight fitting and better protection. FIG. 14 depicts exemplaryembodiments of transparent tubular casing 310 that provides properencapsulation to the solar cell embodiments depicted in FIGS. 4, 7, 8,9, 10, 11 and 13.

Rod or cylindrical shaped solar cells 402, individually encased bytransparent tubular casing 310 (for example those shown in FIGS. 3, 4,7, 8, 9, 10, 13, and 15), can be assembled into solar cell assemblies ofany shape and size. In some embodiments, the assembly can be bifacialarrays 400 (FIG. 4), 700 (FIG. 7), 800 (FIG. 8), 900 (FIG. 9), or 1000(FIG. 10). There is no limit to the number of solar cells 402 in thisplurality (e.g., 10 or more, 100 or more, 1000 or more, 10,000 or more,between 5,000 and one million solar cells 402, etc.).

Alternatively, instead of being encapsulated individually and then beingassembled together for example into planar arrays, solar cells 402 mayalso be encapsulated as arrays. For example, as depicted in FIG. 7C,multiple transparent tubular casings 310 may be manufactured as fusedarrays. This method is advantageous in that little or no additionalconnection between the individual solar cells 402 is required. There isno limit to the number of transparent tubular casings 310 in theassembly as depicted in FIG. 7C (e.g., 10 or more, 100 or more, 1000 ormore, 10,000 or more, between 5,000 and one million transparent tubularcasings 310, etc.). A solar cell assembly is further completed byloading elongated solar cells 402 (for example 402 in FIG. 4A) into allor a portion of the transparent tubular casing 310 in the array oftubular casings.

5.1.3.1 Integrating Solar Cells Having a Filler Layer into TransparentTubular Casings

In some embodiments in accordance with the present invention, a solarcell 402 having a filler layer coated thereon is assembled into atransparent tubular casing 310. In some embodiments in accordance withthe present invention, filler layer 330 comprises one or more of theproperties of: electrical insulation, oxidation eliminating effect,water proofing, and/or physical protection of transparent conductivelayer 412 of solar cell 402 during assembly of solar cell units.

In some embodiments in accordance with the present invention, anelongated solar cell 402, optional filler layer 330, and a transparenttubular casing 310 are assembled using a suction loading methodillustrated in FIG. 20A. Transparent tubular casing 310, made oftransparent glass, plastics or other suitable material, is sealed at oneend 2002. Materials that are used to form filler layer 330, for example,silicone gel, is poured into the sealed transparent tubular casing 310.An example of a silicone gel is Wacker SilGel® 612 (Wacker-Chemie GmbH,Munich, Germany). Wacker SilGel® 612 is a pourable, addition-curing,RTV-2 silicone rubber that vulcanizes at room temperature to a softsilicone gel. Still another example of silicone gel is Sylgard® siliconeelastomer (Dow Corning). Another example of a silicone gel is WackerElastosil® 601 (Wacker-Chemie GmbH, Munich, Germany). Wacker Elastosil®601 is a pourable, addition-curing, RTV-2 silicone rubber. Referring toFIG. 22, silicones can be considered a molecular hybrid between glassand organic linear polymers. As shown in FIG. 22, if there are no Rgroups, only oxygen, the structure is inorganic silica glass (called aQ-type Si). If one oxygen is substituted with an R group (e.g. methyl,ethyl, phenyl, etc.) a resin or silsequioxane (T-type Si) material isformed. These silsequioxanes are more flexible than the Q-typematerials. Finally, if two oxygen atoms are replaced by organic groups avery flexible linear polymer (D-type Si) is obtained. The last structureshown (M-type Si) has three oxygen atoms replaced by R groups, resultingin an end cap structure. Because the backbone chain flexibility isincreasing as R groups are added, the modulus of the materials and theircoefficients of thermal expansion (CTE) also change. In some embodimentsof the present invention the silicone used to form filler layer is aQ-type silicone, a silsequioxane, a D-type silicon, or an M-typesilicon. The elongated solar cell 402 is then loaded into transparenttubular casing 310. Optional suction force may be applied at the openend 2004 of transparent tubular casing 310 to draw the filler materialupwards to completely fill the space between solar cell 402 andtransparent tubular casing 310.

In some embodiments in accordance with the present invention, anelongated solar cell 402, filler layer 330, and a transparent tubularcasing 310 may be assembled using the pressure loading methodillustrated in FIG. 20B. Transparent tubular casing 310, made oftransparent glass, plastics or other suitable material, is dipped incontainer 2008 containing optional filler layer material (e.g., siliconegel) used to form optional filler layer 330. Elongated solar cell 402 isthen loaded into transparent tubular casing 310. Pressure force isapplied at filler material surface 2006 to put the filler materialupwards to completely fill the space between solar cell 402 andtransparent tubular casing 310.

In yet other embodiments in accordance with the present invention, anelongated solar cell 402, filler layer 330 and a transparent tubularcasing 310 is assembled using the pour-and-slide loading method depictedin FIG. 20C. A transparent tubular casing 310, made of transparentglass, plastics or other suitable material, is sealed at one end 2002. Acontainer 2010, containing filler material (e.g., silicone gel), is usedto pour the filler layer material into the sealed transparent tubularcasing 310 while solar cell 402 is simultaneously slid into transparenttubular casing 310. The filler material that is being poured intotransparent tubular casing 310 fills up the space between solar cell 402and transparent tubular casing 310. Advantageously, the filler materialthat is being poured down the side of transparent tubular casing 310provides lubrication to facilitate the slide-loading process.

5.1.3.2 Integrating Solar Cells without an Optional Filler Layer intoTransparent Tubular Casings

In some embodiments in accordance with the present invention, a tubularcasing 310 is assembled onto solar cell 402 without a filler layer 330.In these embodiments, tubular casing 310 may directly contact solar cell402. Tight packing and tubular casing 310 against solar cell 402 may beachieved by using one of the following methods. It will be appreciatedthat the methods for assembling a solar cell unit 300 described in thissection can be used with solar cells 402 that are encased with a fillerlayer 330. However, in such embodiments, layer 330 must be depositedonto transparent conductive layer 412 of solar cells 402 prior tointegrating the solar cell 402 with a transparent tubular casing.

Heat Shrink Loading. In some embodiments, transparent tubular casing 310heat shrinked onto solar cell 402. The heat shrink method may be used toform both plastic and glass transparent tubular casings 310. Forexample, heat-shrinkable plastic tubing made of polyolefin,fluoropolymer (PVC, FEP, PTFE, Kynar® PVDF), chlorinated polyolefin(Neoprene) and highly flexible elastomer (Viton®) heat-shrinkable tubingmay be used to form transparent tubular casing 310. Among suchmaterials, fluoropolymers offer increased lubricity for easy sliding,and low moisture absorption for enhanced dimensional stability. Threesuch materials are commercially accessible: PTFE(polytetrafluoroethylene), FEP (fluorinated ethylene propylene) and PVDF(polyvinylidene fluoride, tradename Kynar®). Transparent heat-shrinkableplastic tubing is available. In some embodiments, the heat shrink tubingis available in an expandable range of 2:1 to 3:1. In some embodiments,the heat shrink ratio of the tubing material is smaller than 2:1, forexample, fluorinated ethylene-propylene (FEP) at 1.3:1. In otherembodiments, a heat shrink tubing suitable for the manufacture oftransparent tubular casing 310 may have heat shrink ratio greater than3:1.

Injection molding to construct transparent tubular casing. In someembodiments, transparent tubular casing 310 may be circumferentiallydisposed onto solar cell 402 by using the method of injection molding. Amore detailed description of the method is already included above. Inthese embodiments, solar cells 402 may be used as the preformed mold andtransparent tubular casing 310 (e.g., made of plastic material) isdirectly formed on the outer surface of solar cells 402. Plasticmaterial does not completely seal molecular water from solar cells 402.Because water interferes with the function of a solar cell 402, it istherefore important to make the solar cell 402 resistant to water. Inthe embodiments where plastic transparent tubular casings 310 are usedto cover solar cells 402, this is accomplished by covering either thesolar cell 402 or transparent tubular casing 310 with one or more layersof transparent water-resistant coating 340 (FIG. 21). In someembodiments, both solar cell 402 and transparent tubular casing 310 arecoated with one or more layers of transparent water-resistant coating340 to extend the functional life time of the solar cell unit 300. Inother embodiments, an optional antireflective coating 350 is alsodisposed on transparent tubular casing 310 to maximize solar cellefficiency.

Liquid Coating Followed by Polymerization. In some embodiments, solarcell 402 is dipped in a liquid-like suspension or resin and subsequentlyexposed to catalyst or curing agent to form transparent tubular casing310 through a polymerization process. In such embodiments, materialsused to form transparent tubular casing 310 comprise silicone,poly-dimethyl siloxane (PDMS), silicone gel, epoxy, acrylics, and anycombination or variation thereof.

5.1.4 Optical and Chemical Properties of the Materials for TransparentTubular Casing and the Optional Filler Layer

In order to maximize input of solar radiation, any layer outside a solarcell 402 (for example, optional filler layer 330 or transparent tubularcasing 310) should not adversely affect the properties of incidentradiation on the solar cell. There are multiple factors to consider inoptimizing the efficiency of solar cells 402. A few significant factorswill be discussed in detail in relation to solar cell production.

Transparency. In order to establish maximized input into solar cellabsorption layer (e.g., semiconductor junction 410), absorption of theincident radiation by any layer outside a solar cell 402 should beavoided or minimized. This transparency requirement varies as a functionof the absorption properties of the underlying semiconductor junction410 of solar cells 402. In general, transparent tubular casing 310 andoptional filler layer 330 should be as transparent as possible to thewavelengths absorbed by the semiconductor junction 410. For example,when the semiconductor junction 410 is based on CIGS, materials used tomake transparent tubular casing 310 and optional filer layer 330 shouldbe transparent to light in the 500 nm to 1200 nm wavelength range.

Ultraviolet Stability. Any material used to construct a layer outsidesolar cell 402 should be chemically stable and, in particular, stableupon exposure to UV radiation. More specifically, such material shouldnot become less transparent upon UV exposure. Ordinary glass partiallyblocks UVA (wavelengths 400 and 300 nm) and it totally blocks UVC andUVB (wavelengths lower than 300 nm). The UV blocking effect of glass isusually due additives, e.g. sodium carbonate, in glass. In someembodiments, additives in transparent tubular casings 310 made of glasscan render the casing 310 entirely UV protective. In such embodiments,because the transparent tubular casing 310 provides complete protectionfrom UV wavelengths, the UV stability requirements of the underlyingoptional filler layer 330 are reduced. For example, EVA, PVB, TPU(urethane), silicones, polycarbonates, and acrylics can be adapted toform a filler layer 330 when transparent tubular casing 310 is made ofUV protective glass. Alternatively, in some embodiments, wheretransparent tubular casing 310 is made of plastic material, UV stabilityrequirement should be strictly followed.

Plastic materials that are sensitive to UV radiation should not be usedas transparent tubular casing 310 because yellowing of the materialand/or optional filler layer 330 blocks radiation input into the solarcells 402 and reduces their efficiency. In addition, cracking oftransparent tubular casing 310 due to UV exposure permanently damagessolar cells 402. For example, fluoropolymers like ETFE, and THV (Dyneon)are UV stable and highly transparent, while PET is transparent, but notsufficiently UV stable. In some embodiments, transparent tubular casing310 is made of fluoropolymer based on monomers of tetrafluoroethylene,hexafluoropropylene and vinylidene fluoride. In addition, polyvinylchloride (“PVC” or “vinyl”), one of the most common synthetic materials,is also sensitive to UV exposure. Methods have been developed to renderPVC UV-stabilized, but even UV stabilized PVC is typically notsufficiently durable (for example, yellowing and cracking of PVC productwill occur over relative short term usage). Urethanes are better suited,but depend on the exact chemical nature of the polymer backbone.Urethane material is stable when the polymer backbone is formed by lessreactive chemical groups (e.g., aliphatic or aromatic). On the otherhand when the polymer backbone is formed by more reactive groups (e.g.,double bonds), yellowing of the material occurs as a result ofUV-catalyzed breakdown of the double bonds. Similarly, EVA will yellowand so will PVB upon continued exposure to UV light. Other options arepolycarbonate (can be stabilized against UV for up to 10 years ODexposure) or acrylics (inherently UV stable).

Reflective Properties. Referring to FIG. 21, an incident beam L₁ hitsthe surface of transparent tubular casing 310. Part of the incident beamL₁ is reflected as L₂ while the remainder of incident beam L₁ (e.g., asrefracted beam L₃ in FIG. 21) travels through transparent tubular casing310. In some embodiments in accordance with the present invention, therefracted beam L₃ directly hits transparent conductive layer 412 ofsolar cell 402 (e.g., when optional filler layer 330 is absent).Alternatively, when filler layer 330 is present, as depicted in FIG. 21,L₃ hits the outer surface of the filler layer 330, and the processes ofreflection and refraction is repeated as it was when L₁ hit the surfaceof transparent tubular casing 310, with some of L₃ reflected into fillerlayer 330 and some of L₃ refracted by filler layer 330.

In order to maximize input of solar radiation, reflection at the outersurface of transparent tubular casing 310 should be minimized.Antireflective coating, either as a separate layer 350 or in combinationwith the water resistant coating 340, may be applied on the outside oftransparent tubular casing 310. In some embodiments, this antireflectivecoating is made of MgF₂. In some embodiments, this antireflectivecoating is made of silicone nitrate or titanium nitrate. In otherembodiments, this antireflective coating is made of one or more layersof silicon monoxide (SiO). For example, shiny silicon can act as amirror and reflects more than thirty percent of the light that shines onit. A single layer of SiO reduces surface reflection to about tenpercent, and a second layer of SiO can lower the reflection to less thanfour percent. Other organic antireflective materials, in particular, onewhich prevents back reflection from the surface of or lower layers inthe semiconductor device and eliminates the standing waves andreflective notching due to various optical properties of lower layers onthe wafer and the photosensitive film, are disclosed in U.S. Pat. No.6,803,172, which is hereby incorporated by reference herein in itsentirety. Additional antireflective coating materials and methods aredisclosed in U.S. Pat. Nos. 6,689,535; 6,673,713; 6,635,583; 6,784,094;and 6,713,234, each of which is hereby incorporated herein by referencein its entirety.

Alternatively, the outer surface of transparent tubular casing 310 maybe textured to reduce reflected radiation. Chemical etching creates apattern of cones and pyramids, which capture light rays that mightotherwise be deflected away from the cell. Reflected light is redirecteddown into the cell, where it has another chance to be absorbed. Materialand methods for creating an anti-reflective layer by etching or by acombination of etching and coating techniques are disclosed in U.S. Pat.Nos. 6,039,888; 6,004,722; and 6,221,776; each of which is herebyincorporated herein by reference in its entirety.

Refractive Properties. As depicted in FIG. 21, part of incident beam L₁is refracted as refracted beam L₃. How much and to which directionincident beam L₁ is bent from its path is determined by the refractiveindices of the media in which beams L₁ and L₃ travel. Snell's lawspecifies:η₁ sin(θ₁)=η₂ sin(θ₂),where η₁ and η₂ are the refractive indices of the two bordering media 1and 2 while θ₁ and θ₂ represent the angle of incidence and the angle ofrefraction, respectively.

In FIG. 21, the first refraction process occurs when incident beam L₁travels from air through transparent tubular casing 310 as L₃. Ambientair has a refractive index around 1 (vacuum space has a refractive indexof 1, which is the smallest among all known materials), which is muchsmaller than the refractive index of glass material (ranging from 1.4 to1.9 with the commonly used material having refractive indices around1.5) or plastic material (around 1.45). Because η_(air) is always muchsmaller than η₃₁₀ whether tubular casing is formed by glass or plasticmaterial, the refractive angle η₃₁₀ is always much smaller than theincident angle θ_(air), i.e., the incident beam is always bent towardssolar cell 402 as it travels through transparent tubular casing 310.

In the presence of a filler layer 330, beam L₃ becomes the new incidentbeam when it travels through the filler layer 330. Ideally, according toSnell's law and the preceding analysis, the refractive index of thefiller layer 330 (e.g., η₃₁₀ in FIG. 21) should be larger than therefractive index of transparent tubular casing 310 so that the refractedbeam of incident beam L₃ will also be bent towards solar cell 402. Inthis ideal situation, every incident beam on transparent tubular casing310 will be bent towards solar cell 402 after two reflection processes.In practice, however, optional filler layer 330 is made of a fluid-likematerial (albeit sometimes very viscous fluid-like material) such thatloading of solar cells 402 into transparent tubular casing 310 may beachieved as described above. In practice, efficient solar radiationabsorption is achieved by choosing filler material that has refractiveindex close to those of transparent tubular casing 310. In someembodiments, materials that form transparent tubular casing 310 comprisetransparent materials (either glass or plastic or other suitablematerials) with refractive indices around 1.5. For example, fused silicaglass has a refractive index of 1.46. Borosilicate glass materials haverefractive indices between 1.45 and 1.55 (e.g., Pyrex® glass has arefractive index of 1.47). Flint glass materials with various amounts oflead additive have refractive indices between 1.5 and 1.9. Commonplastic materials have refractive indices between 1.46 and 1.55.

Exemplary materials with the appropriate optical properties for formingfiller layer 330 further comprise silicone, polydimethyl siloxane(PDMS), silicone gel, epoxy, and acrylic material. Becausesilicone-based adhesives and sealants have a high degree of flexibility,they lack the strength of other epoxy or acrylic resins. Transparenttubular casing 310, optional filler layer 330, optional antireflectivelayer 350, water-resistant layer 340, or any combination thereof form apackage to maximize and maintain solar cell 402 efficiency, providephysical support, and prolong the life time of solar cell units 402.

In some embodiments, glass, plastic, epoxy or acrylic resin may be usedto form transparent tubular casing 310. In some embodiments, optionalantireflective 350 and/or water resistant coating 340 arecircumferentially disposed on transparent tubular casing 310. In somesuch embodiments, filler layer 330 is formed by softer and more flexibleoptically suitable material such as silicone gel. For example, in someembodiments, filler layer 330 is formed by a silicone gel such as asilicone-based adhesives or sealants. In some embodiments, filler layer330 is formed by GE RTV 615 Silicone. RTV 615 is an optically clear,two-part flowable silicone product that requires SS4120 as primer forpolymerization. (RTV615-1P), both available from General Electric(Fairfield, Conn.). Silicone-based adhesives or sealants are based ontough silicone elastomeric technology. The characteristics ofsilicone-based materials, such as adhesives and sealants, are controlledby three factors: resin mixing ratio, potting life and curingconditions.

Advantageously, silicone adhesives have a high degree of flexibility andvery high temperature resistance (up to 600° F.). Silicone-basedadhesives and sealants have a high degree of flexibility. Silicone-basedadhesives and sealants are available in a number of technologies (orcure systems). These technologies include pressure sensitive, radiationcured, moisture cured, thermo-set and room temperature vulcanizing(RTV). In some embodiments, the silicone-based sealants usetwo-component addition or condensation curing systems or singlecomponent (RTV) forms. RTV forms cure easily through reaction withmoisture in the air and give off acid fumes or other by-product vaporsduring curing.

Pressure sensitive silicone adhesives adhere to most surfaces with veryslight pressure and retain their tackiness. This type of material formsviscoelastic bonds that are aggressively and permanently tacky, andadheres without the need of more than finger or hand pressure. In someembodiments, radiation is used to cure silicone-based adhesives. In someembodiments, ultraviolet light, visible light or electron beanirradiation is used to initiate curing of sealants, which allows apermanent bond without heating or excessive heat generation. WhileUV-based curing requires one substrate to be UV transparent, theelectron beam can penetrate through material that is opaque to UV light.Certain silicone adhesives and cyanoacrylates based on a moisture orwater curing mechanism may need additional reagents properly attached tothe solar cell 402 without affecting the proper functioning of solarcells 402. Thermo-set silicone adhesives and silicone sealants arecross-linked polymeric resins cured using heat or heat and pressure.Cured thermo-set resins do not melt and flow when heated, but they maysoften. Vulcanization is a thermosetting reaction involving the use ofheat and/or pressure in conjunction with a vulcanizing agent, resultingin greatly increased strength, stability and elasticity in rubber-likematerials. RTV silicone rubbers are room temperature vulcanizingmaterials. The vulcanizing agent is a cross-linking compound orcatalyst. In some embodiments in accordance with the present invention,sulfur is added as the traditional vulcanizing agent.

In some embodiments, for example, when optional filler layer 330 isabsent, epoxy or acrylic material may be applied directly over solarcell 402 to form transparent tubular casing 310 directly. In suchembodiments, care is taken to ensure that the non-glass transparenttubular casing 310 is also equipped with water resistant and/orantireflective properties to ensure efficient operation over areasonable period of usage time.

Electrical Insulation. An important characteristics of transparenttubular casing 310 and optional filler layer 330 is that these layersshould provide complete electrical insulation. No conductive materialshould be used to form either transparent tubular casing 310 or optionalfiller layer 330.

Dimension requirement. The combined width of each of the layers outsidesolar cell 402 (e.g., the combination of transparent tubular casing 310and/or optional filler layer 330) in some embodiments is:$r_{i} \geq \frac{r_{o}}{\eta_{{outer}\quad{ring}}}$where, referring to FIG. 3B,

r_(i) is the radius of solar cell 402, assuming that semiconductorjunction 410 is a thin-film junction;

r_(o) is the radius of the outermost layer of transparent tubular casing310 and/or optional filler layer 330; and

η_(outer ring) is the refractive index of the outermost layer oftransparent tubular casing 310 and/or optional filler layer 330.

As noted above, the refractive index of many of the materials used tomake transparent tubular casing 310 and/or optional filler layer 330 isabout 1.5. Thus, in typical embodiments, values of r_(o) are permissiblethat are less than 1.5*r_(i). This constraint places a boundary onallowable thickness for the combination of transparent tubular casing310 and/or optional filler layer 330.

5.1.3.5 Additional Methods for Forming Transparent Tubular Casing

In some embodiments, transparent tubular casing 310 is formed on anunderlying layer (e.g., is formed on transparent conductive layer 412,filler layer 330 or a water resistant layer) by spin coating, dipcoating, plastic spraying, casting, Doctor's blade or tape casting, glowdischarge polymerization, or UV curing. These technologies are discussedin greater detail in Madou, Fundamentals of Microfabrication, Chapter 3,pp. 159-161, second edition, CRC Press, New York, 2002, which is herebyincorporated by reference in its entirety. Casting is particularlysuitable in instances where transparent tubular casing 310 is formedfrom acrylics or polycarbonates. UV curing is particularly suitable ininstances where transparent tubular casing 310 is formed from anacrylic.

5.2 Exemplary Semiconductor Junctions

Referring to FIG. 5A, in one embodiment, semiconductor junction 410 is aheterojunction between an absorber layer 502, disposed on back-electrode404, and a junction partner layer 504, disposed on absorber layer 502.Layers 502 and 504 are composed of different semiconductors withdifferent band gaps and electron affinities such that junction partnerlayer 504 has a larger band gap than absorber layer 502. In someembodiments, absorber layer 502 is p-doped and junction partner layer504 is n-doped. In such embodiments, transparent conductive layer 412 isn⁺-doped. In alternative embodiments, absorber layer 502 is n-doped andjunction partner layer 504 is p-doped. In such embodiments, transparentconductive layer 412 is p⁺-doped. In some embodiments, thesemiconductors listed in Pandey, Handbook of SemiconductorElectrodeposition, Marcel Dekker Inc., 1996, Appendix 5, which is herebyincorporated by reference herein in its entirety, are used to formsemiconductor junction 410.

5.2.1 Thin-Film Semiconductor Junctions Based on Copper IndiumDiselenide and Other Type I-III-VI Materials

Continuing to refer to FIG. 5A, in some embodiments, absorber layer 502is a group I-III′-VI₂ compound such as copper indium di-selenide(CuInSe₂; also known as CIS). In some embodiments, absorber layer 502 isa group I-III-VI₂ ternary compound selected from the group consisting ofCdGeAs₂, ZnSnAs₂, CuInTe₂, AgInTe₂, CuInSe₂, CuGaTe₂, ZnGeAs₂, CdSnP₂,AgInSe₂, AgGaTe₂, CuInS₂, CdSiAs₂, ZnSnP₂, CdGeP₂, ZnSnAs₂, CuGaSe₂,AgGaSe₂, AgInS₂, ZnGeP₂, ZnSiAs₂, ZnSiP₂, CdSiP₂, or CuGaS₂ of eitherthe p-type or the n-type when such compound is known to exist.

In some embodiments, junction partner layer 504 is CdS, ZnS, ZnSe, orCdZnS. In one embodiment, absorber layer 502 is p-type CIS and junctionpartner layer 504 is n⁻ type CdS, ZnS, ZnSe, or CdZnS. Suchsemiconductor junctions 410 are described in Chapter 6 of Bube,Photovoltaic Materials, 1998, Imperial College Press, London, which ishereby incorporated by reference in its entirety. Such semiconductorjunctions 410 are described in Chapter 6 of Bube, PhotovoltaicMaterials, 1998, Imperial College Press, London, which is herebyincorporated by reference in its entirety.

In some embodiments, absorber layer 502 iscopper-indium-gallium-diselenide (CIGS). Such a layer is also known asCu(InGa)Se₂. In some embodiments, absorber layer 502 iscopper-indium-gallium-diselenide (CIGS) and junction partner layer 504is CdS, ZnS, ZnSe, or CdZnS. In some embodiments, absorber layer 502 isp-type CIGS and junction partner layer 504 is n-type CdS, ZnS, ZnSe, orCdZnS. Such semiconductor junctions 410 are described in Chapter 13 ofHandbook of Photovoltaic Science and Engineering, 2003, Luque andHegedus (eds.), Wiley & Sons, West Sussex, England, Chapter 12, which ishereby incorporated by reference in its entirety. In some embodiments,CIGS is deposited using techniques disclosed in Beck and Britt, FinalTechnical Report, January 2006, NREL/SR-520-39119; and Delahoy and Chen,August 2005, “Advanced CIGS Photovoltaic Technology,” subcontractreport; Kapur et al., January 2005 subcontract report,NREL/SR-520-37284, “Lab to Large Scale Transition for Non-Vacuum ThinFilm CIGS Solar Cells”; Simpson et al., October 2005 subcontract report,“Trajectory-Oriented and Fault-Tolerant-Based Intelligent ProcessControl for Flexible CIGS PV Module Manufacturing,” NREL/SR-520-38681;and Ramanathan et al., 31^(st) IEEE Photovoltaics Specialists Conferenceand Exhibition, Lake Buena Vista, Fla., Jan. 3-7, 2005, each of which ishereby incorporated by reference herein in its entirety.

In some embodiments CIGS absorber layer 502 is grown on a molybdenumback-electrode 404 by evaporation from elemental sources in accordancewith a three stage process described in Ramanthan et al., 2003,“Properties of 19.2% Efficiency ZnO/CdS/CuInGaSe₂ Thin-film SolarCells,” Progress in Photovoltaics: Research and Applications 11, 225,which is hereby incorporated by reference herein in its entirety. Insome embodiments layer 504 is a ZnS(O,OH) buffer layer as described, forexample, in Ramanathan et al., Conference Paper, “CIGS Thin-Film SolarResearch at NREL: FY04 Results and Accomplishments,” NREL/CP-520-37020,January 2005, which is hereby incorporated by reference herein in itsentirety.

In some embodiments, layer 502 is between 0.5 μm and 2.0 μm thick. Insome embodiments, the composition ratio of Cu/(In+Ga) in layer 502 isbetween 0.7 and 0.95. In some embodiments, the composition ratio ofGa/(In+Ga) in layer 502 is between 0.2 and 0.4. In some embodiments theCIGS absorber has a <110> crystallographic orientation. In someembodiments the CIGS absorber has a <112> crystallographic orientation.In some embodiments the CIGS absorber is randomly oriented.

5.2.2 Semiconductor Junctions Based on Amorphous Silicon orPolycrystalline Silicon

In some embodiments, referring to FIG. 5B, semiconductor junction 410comprises amorphous silicon. In some embodiments this is an n/n typeheterojunction. For example, in some embodiments, layer 514 comprisesSnO₂(Sb), layer 512 comprises undoped amorphous silicon, and layer 510comprises n+ doped amorphous silicon.

In some embodiments, semiconductor junction 410 is a p-i-n typejunction. For example, in some embodiments, layer 514 is p⁺ dopedamorphous silicon, layer 512 is undoped amorphous silicon, and layer 510is n⁺ amorphous silicon. Such semiconductor junctions 410 are describedin Chapter 3 of Bube, Photovoltaic Materials, 1998, Imperial CollegePress, London, which is hereby incorporated by reference in itsentirety.

In some embodiments of the present invention, semiconductor junction 410is based upon thin-film polycrystalline. Referring to FIG. 5B, in oneexample in accordance with such embodiments, layer 510 is a p-dopedpolycrystalline silicon, layer 512 is depleted polycrystalline siliconand layer 514 is n-doped polycrystalline silicon. Such semiconductorjunctions are described in Green, Silicon Solar Cells: AdvancedPrinciples & Practice, Centre for Photovoltaic Devices and Systems,University of New South Wales, Sydney, 1995; and Bube, PhotovoltaicMaterials, 1998, Imperial College Press, London, pp. 57-66, which ishereby incorporated by reference in its entirety.

In some embodiments of the present invention, semiconductor junctions410 based upon p-type microcrystalline Si:H and microcrystalline Si:C:Hin an amorphous Si:H solar cell are used. Such semiconductor junctionsare described in Bube, Photovoltaic Materials, 1998, Imperial CollegePress, London, pp. 66-67, and the references cited therein, which ishereby incorporated by reference in its entirety.

In some embodiments, of the present invention, semiconductor junction410 is a tandem junction. Tandem junctions are described in, forexample,

Kim et al., 1989, “Lightweight (AlGaAs)GaAs/CuInSe2 tandem junctionsolar cells for space applications,” Aerospace and Electronic SystemsMagazine, IEEE Volume 4, Issue 11, November 1989 Page(s): 23-32; Deng,2005, “Optimization of a-SiGe based triple, tandem and single-junctionsolar cells Photovoltaic Specialists Conference, 2005 Conference Recordof the Thirty-first IEEE 3-7 Jan. 2005 Page(s): 1365-1370; Arya et al.,2000, Amorphous silicon based tandem junction thin-film technology: amanufacturing perspective,” Photovoltaic Specialists Conference, 2000.Conference Record of the Twenty-Eighth IEEE 15-22 Sep. 2000 Page(s):1433-1436; Hart, 1988, “High altitude current-voltage measurement ofGaAs/Ge solar cells,” Photovoltaic Specialists Conference, 1988,Conference Record of the Twentieth IEEE 26-30 Sep. 1988 Page(s): 764-765vol. 1; Kim, 1988, “High efficiency GaAs/CuInSe2 tandem junction solarcells,” Photovoltaic Specialists Conference, 1988, Conference Record ofthe Twentieth IEEE 26-30 Sep. 1988 Page(s): 457-461 vol. 1; Mitchell,1988, “Single and tandem junction CuInSe2 cell and module technology,”Photovoltaic Specialists Conference, 1988., Conference Record of theTwentieth IEEE 26-30 Sep. 1988 Page(s): 1384-1389 vol. 2; and Kim, 1989,“High specific power (AlGaAs)GaAs/CuInSe2 tandem junction solar cellsfor space applications,” Energy Conversion Engineering Conference, 1989,IECEC-89, Proceedings of the 24^(th) Intersociety 6-11 Aug. 1989Page(s): 779-784 vol. 2, each of which is hereby incorporated byreference herein in its entirety.

5.2.3 Semiconductor Junctions Based on Gallium Arsenide and Other TypeIII-V Materials

In some embodiments, semiconductor junctions 410 are based upon galliumarsenide (GaAs) or other III-V materials such as InP, AlSb, and CdTe.GaAs is a direct-band gap material having a band gap of 1.43 eV and canabsorb 97% of AM1 radiation in a thickness of about two microns.Suitable type III-V junctions that can serve as semiconductor junctions410 of the present invention are described in Chapter 4 of Bube,Photovoltaic Materials, 1998, Imperial College Press, London, which ishereby incorporated by reference in its entirety.

Furthermore, in some embodiments semiconductor junction 410 is a hybridmultijunction solar cell such as a GaAs/Si mechanically stackedmultijunction as described by Gee and Virshup, 1988, 20^(th) IEEEPhotovoltaic Specialist Conference, IEEE Publishing, New York, p. 754,which is hereby incorporated by reference herein in its entirety, aGaAs/CuInSe₂ MSMJ four-terminal device, consisting of a GaAs thin filmtop cell and a ZnCdS/CuInSe₂ thin bottom cell described by Stanbery etal., 19 ^(th) IEEE Photovoltaic Specialist Conference, IEEE Publishing,New York, p. 280, and Kim et al., 20^(th) IEEE Photovoltaic SpecialistConference, IEEE Publishing, New York, p. 1487, each of which is herebyincorporated by reference herein in its entirety. Other hybridmultijunction solar cells are described in Bube, Photovoltaic Materials,1998, Imperial College Press, London, pp. 131-132, which is herebyincorporated by reference herein in its entirety.

5.2.4 Semiconductor Junctions Based on Cadmium Telluride and Other TypeII-VI Materials

In some embodiments, semiconductor junctions 410 are based upon II-VIcompounds that can be prepared in either the n-type or the p-type form.Accordingly, in some embodiments, referring to FIG. 5C, semiconductorjunction 410 is a p-n heterojunction in which layers 520 and 540 are anycombination set forth in the following table or alloys thereof. Layer520 Layer 540 n-CdSe p-CdTe n-ZnCdS p-CdTe n-ZnSSe p-CdTe p-ZnTe n-CdSen-CdS p-CdTe n-CdS p-ZnTe p-ZnTe n-CdTe n-ZnSe p-CdTe n-ZnSe p-ZnTen-ZnS p-CdTe n-ZnS p-ZnTeMethods for manufacturing semiconductor junctions 410 are based uponII-VI compounds are described in Chapter 4 of Bube, PhotovoltaicMaterials, 1998, Imperial College Press, London, which is herebyincorporated by reference in its entirety.

5.2.5 Semiconductor Junctions Based on Crystalline Silicon

While semiconductor junctions 410 that are made from thin filmsemiconductor films are preferred, the invention is not so limited. Insome embodiments semiconductor junctions 410 is based upon crystallinesilicon. For example, referring to FIG. 5D, in some embodiments,semiconductor junction 410 comprises a layer of p-type crystallinesilicon 540 and a layer of n-type crystalline silicon 550. Methods formanufacturing crystalline silicon semiconductor junctions 410 aredescribed in Chapter 2 of Bube, Photovoltaic Materials, 1998, ImperialCollege Press, London, which is hereby incorporated by reference hereinin its entirety.

5.3 Albedo Embodiments

The solar cell design of the present invention is advantageous becauseit can collect light through the entire circumferential surface.Accordingly, in some embodiments of the present invention, these solarcell assemblies (e.g., solar cell assembly 400, 700, 800, 900, etc.) arearranged in a reflective environment in which surfaces around the solarcell assembly have some amount of albedo. Albedo is a measure ofreflectivity of a surface or body. It is the ratio of electromagneticradiation (EM radiation) reflected to the amount incident upon it. Thisfraction is usually expressed as a percentage from 0% to 100%. In someembodiments, surfaces in the vicinity of the solar cell assemblies ofthe present invention are prepared so that they have a high albedo bypainting such surfaces a reflective white color. In some embodiments,other materials that have a high albedo can be used. For example, thealbedo of some materials around such solar cells approach or exceedninety percent. See, for example, Boer, 1977, Solar Energy 19, 525,which is hereby incorporated by reference herein in its entirety.However, surfaces having any amount of albedo (e.g., five percent ormore, ten percent or more, twenty percent or more) are within the scopeof the present invention. In one embodiment, the solar cells assembliesof the present invention are arranged in rows above a gravel surface,where the gravel has been painted white in order to improve thereflective properties of the gravel. In general, any Lambertian ordiffuse reflector surface can be used to provide a high albedo surface.

By way of example, in some embodiments of the present invention, thebifacial solar cell assemblies (panels) of the present invention have afirst and second face and are placed in rows facing South in theNorthern hemisphere (or facing North in the Southern hemisphere). Eachof the panels is placed some distance above the ground (e.g., 100 cmabove the ground). The East-West separation between the panels issomewhat dependent upon the overall dimensions of the panels. By way ofillustration only, panels having overall dimensions of about 106 cm×44cm are placed in the rows such that the East-West separation between thepanels is between 10 cm and 50 cm. In one specific example the East-Westseparation between the panels is 25 cm.

In some embodiments, the central point of the panels in the rows ofpanels is between 0.5 meters and 2.5 meters from the ground. In onespecific example, the central point of the panels is 1.55 meters fromthe ground. The North-South separation between the rows of panels isdependent on the dimensions of the panels. By way of illustration, inone specific example, in which the panels have overall dimensions ofabout 106 cm×44 cm, the North-South separation is 2.8 meters. In someembodiments, the North-South separation is between 0.5 meters and 5meters. In some embodiments, the North-South separation is between 1meter and 3 meters.

In some embodiments, models for computing the amount of sunlightreceived by solar panels as put forth in Lorenzo et al., 1985, SolarCells 13, pp. 277-292, which is hereby incorporated by reference hereinin its entirety, are used to compute the optimum horizontal tilt andEast-West separation of the solar panels in the rows of solar panelsthat are placed in a reflective environment. In some embodiments,internal or external reflectors are implemented in the solar cellassembly to take advantage of the albedo effect and enhance light inputinto the solar cell assembly. An exemplary embodiment of the internalreflectors (e.g., reflector 1404) is depicted in FIG. 16. Moredescription of albedo surfaces that can be used in conjunction with thepresent invention are disclosed in U.S. patent application Ser. No.11/315,523, which is hereby incorporated by reference in its entirety.

5.4 Dual Layer Core Embodiments

Embodiments of the present invention in which conductive core 404 of thesolar cells 402 of the present invention is made of a uniform conductivematerial have been disclosed. The invention is not limited to theseembodiments. In some embodiments, conductive core 404 in fact has aninner core and an outer conductive core. The inner core can be referredto as a substrate 403 while the outer core can be referred to asback-electrode 404 in such embodiment. In such embodiments, the outerconductive core is circumferentially disposed on substrate 403. In suchembodiments, substrate 403 is typically nonconductive whereas the outercore is conductive. Substrate 403 has an elongated shape consistent withother embodiments of the present invention. For instance, in oneembodiment, substrate 403 is made of glass fibers in the form of a wire.In some embodiments, substrate 403 is an electrically conductivenonmetallic material. However, the present invention is not limited toembodiments in which substrate 403 is electrically conductive becausethe outer core can function as the electrode. In some embodiments,substrate 403 is tubing (e.g., plastic or glass tubing).

In some embodiments, substrate 403 is made of a material such aspolybenzamidazole (e.g., Celazole®, available from Boedeker Plastics,Inc., Shiner, Tex.). In some embodiments, the inner core is made ofpolymide (e.g., DuPont™ Vespel®, or DuPont™ Kapton®, Wilmington, Del.).In some embodiments, the inner core is made of polytetrafluoroethylene(PTFE) or polyetheretherketone (PEEK), each of which is available fromBoedeker Plastics, Inc. In some embodiments, substrate 403 is made ofpolyamide-imide (e.g., Torlon® PAI, Solvay Advanced Polymers,Alpharetta, Ga.).

In some embodiments, substrate 403 is made of a glass-based phenolic.Phenolic laminates are made by applying heat and pressure to layers ofpaper, canvas, linen or glass cloth impregnated with syntheticthermosetting resins. When heat and pressure are applied to the layers,a chemical reaction (polymerization) transforms the separate layers intoa single laminated material with a “set” shape that cannot be softenedagain. Therefore, these materials are called “thermosets.” A variety ofresin types and cloth materials can be used to manufacture thermosetlaminates with a range of mechanical, thermal, and electricalproperties. In some embodiments, substrate 403 is a phenoloic laminatehaving a NEMA grade of G-3, G-5, G-7, G-9, G-10 or G-11. Exemplaryphenolic laminates are available from Boedeker Plastics, Inc.

In some embodiments, substrate 403 is made of polystyrene. Examples ofpolystyrene include general purpose polystyrene and high impactpolystyrene as detailed in Marks' Standard Handbook for MechanicalEngineers, ninth edition, 1987, McGraw-Hill, Inc., p. 6-174, which ishereby incorporated by reference herein in its entirety. In still otherembodiments, substrate 403 is made of cross-linked polystyrene. Oneexample of cross-linked polystyrene is Rexolite® (C-Lec Plastics, Inc).Rexolite is a thermoset, in particular a rigid and translucent plasticproduced by cross linking polystyrene with divinylbenzene.

In some embodiments, substrate 403 is a polyester wire (e.g., a Mylar®wire). Mylar® is available from DuPont Teijin Films (Wilmington, Del.).In still other embodiments, substrate 403 is made of Durastone®, whichis made by using polyester, vinylester, epoxid and modified epoxy resinscombined with glass fibers (Roechling Engineering Plastic Pte Ltd.,Singapore).

In still other embodiments, substrate 403 is made of polycarbonate. Suchpolycarbonates can have varying amounts of glass fibers (e.g., 10%, 20%,30%, or 40%) in order to adjust tensile strength, stiffness, compressivestrength, as well as the thermal expansion coefficient of the material.Exemplary polycarbonates are Zelux® M and Zelux® W, which are availablefrom Boedeker Plastics, Inc.

In some embodiments, substrate 403 is made of polyethylene. In someembodiments, substrate 403 is made of low density polyethylene (LDPE),high density polyethylene (HDPE), or ultra high molecular weightpolyethylene (UHMW PE). Chemical properties of HDPE are described inMarks' Standard Handbook for Mechanical Engineers, ninth edition, 1987,McGraw-Hill, Inc., p. 6-173, which is hereby incorporated by referenceherein in its entirety. In some embodiments, substrate 403 is made ofacrylonitrile-butadiene-styrene, polytetrafluoro-ethylene (Teflon),polymethacrylate (lucite or plexiglass), nylon 6,6, cellulose acetatebutyrate, cellulose acetate, rigid vinyl, plasticized vinyl, orpolypropylene. Chemical properties of these materials are described inMarks' Standard Handbook for Mechanical Engineers, ninth edition, 1987,McGraw-Hill, Inc., pp. 6-172 through 1-175, which is hereby incorporatedby reference herein in its entirety.

Additional exemplary materials that can be used to form substrate 403are found in Modern Plastics Encyclopedia, McGraw-Hill; ReinholdPlastics Applications Series, Reinhold Roff, Fibres, Plastics andRubbers, Butterworth; Lee and Neville, Epoxy Resins, McGraw-Hill;Bilmetyer, Textbook of Polymer Science, Interscience; Schmidt andMarlies, Principles of high polymer theory and practice, McGraw-Hill;Beadle (ed.), Plastics, Morgan-Grampiand, Ltd., 2 vols. 1970; Tobolskyand Mark (eds.), Polymer Science and Materials, Wiley, 1971; Glanville,The Plastics's Engineer's Data Book, Industrial Press, 1971; Mohr(editor and senior author), Oleesky, Shook, and Meyers, SPI Handbook ofTechnology and Engineering of Reinforced Plastics Composites, VanNostrand Reinhold, 1973, each of which is hereby incorporated byreference herein in its entirety.

In general, outer core is made out of any material that can support thephotovoltaic current generated by solar cell with negligible resistivelosses. In some embodiments, outer core is made of any conductive metal,such as aluminum, molybdenum, steel, nickel, silver, gold, or an alloythereof. In some embodiments, outer core is made out of a metal-,graphite-, carbon black-, or superconductive carbon black-filled oxide,epoxy, glass, or plastic. In some embodiments, outer core is made of aconductive plastic. In some embodiments, this conductive plastic isinherently conductive without any requirement for a filler. In someembodiments, inner core is made out of a conductive material and outercore is made out of molybdenum. In some embodiments, inner core is madeout of a nonconductive material, such as a glass rod, and outer core ismade out of molybdenum.

5.5 Exemplary Dimensions

The present invention encompasses solar cell assemblies having anydimensions that fall within a broad range of dimensions. For example,referring to FIG. 4B, the present invention encompasses solar cellassemblies having a length l between 1 cm and 50,000 cm and a width wbetween 1 cm and 50,000 cm. In some embodiments, the solar cellassemblies have a length l between 10 cm and 1,000 cm and a width wbetween 10 cm and 1,000 cm. In some embodiments, the solar cellassemblies have a length l between 40 cm and 500 cm and a width wbetween 40 cm and 500 cm.

5.6 Additional Solar Cell Embodiments

Using FIG. 3B for reference to element numbers, in some embodiments,copper-indium-gallium-diselenide (Cu(InGa)Se₂), referred to herein asCIGS, is used to make the absorber layer of junction 10. In suchembodiments, back-electrode 404 can be made of molybdenum. In someembodiments, back-electrode 404 comprises an inner core of polyimide andan outer core that is a thin film of molybdenum sputtered onto thepolyimide core prior to CIGS deposition. On top of the molybdenum, theCIGS film, which absorbs the light, is evaporated. Cadmium sulfide (CdS)is then deposited on the CIGS in order to complete semiconductorjunction 410. Optionally, a thin intrinsic layer (i-layer) 415 is thendeposited on the semiconductor junction 410. The i-layer 415 can beformed using a material including but not limited to, zinc oxide, metaloxide or any transparent material that is highly insulating. Next,transparent conductive layer 412 is disposed on either the i-layer (whenpresent) or the semiconductor junction 410 (when the i-layer is notpresent). Transparent conductive layer 412 can be made of a materialsuch as aluminum doped zinc oxide (ZnO:Al), gallium doped zinc oxide,boron dope zinc oxide, indium-zinc oxide, or indium-tin oxide.

ITN Energy Systems, Inc., Global Solar Energy, Inc., and the Instituteof Energy Conversion (IEC), have collaboratively developed technologyfor manufacturing CIGS photovoltaics on polyimide substrates using aroll-to-roll co-evaporation process for deposition of the CIGS layer. Inthis process, a roll of molybdenum-coated polyimide film (referred to asthe web) is unrolled and moved continuously into and through one or moredeposition zones. In the deposition zones, the web is heated totemperatures of up to ˜450° C. and copper, indium, and gallium areevaporated onto it in the presence of selenium vapor. After passing outof the deposition zone(s), the web cools and is wound onto a take-upspool. See, for example, 2003, Jensen et al., “Back Contact CrackingDuring Fabrication of CIGS Solar Cells on Polyimide Substrates,” NCPVand Solar Program Review Meeting 2003, NREL/CD-520-33586, pages 877-881,which is hereby incorporated by reference in its entirety. Likewise,Birkmire et al., 2005, Progress in Photovoltaics: Research andApplications 13, 141-148, hereby incorporated by reference, disclose apolyimide/Mo web structure, specifically,PI/Mo/Cu(InGa)Se₂/CdS/ZnO/ITO/Ni—Al. Deposition of similar structures onstainless foil has also been explored. See, for example, Simpson et al.,2004, “Manufacturing Process Advancements for Flexible CIGS PV onStainless Foil,” DOE Solar Energy Technologies Program Review Meeting,PV Manufacturing Research and Development, P032, which is herebyincorporated by reference herein in its entirety.

In some embodiments of the present invention, an absorber material isdeposited onto a polyimide/molybdenum web, such as those developed byGlobal Solar Energy (Tucson, Ariz.), or a metal foil (e.g., the foildisclosed in Simpson et al.). In some embodiments, the absorber materialis any of the absorbers disclosed herein. In a particular embodiment,the absorber is Cu(InGa)Se₂. In some embodiments, the elongated core ismade of a nonconductive material such as undoped plastic. In someembodiments, the elongated core is made of a conductive material such asa conductive metal, a metal-filled epoxy, glass, or resin, or aconductive plastic (e.g., a plastic containing a conducting filler).Next, semiconductor junction 410 is completed by depositing a windowlayer onto the absorber layer. In the case where the absorber layer isCu(InGa)Se₂, CdS can be used. Finally, optional i-layer 415 andtransparent conductive layer 412 are added to complete the solar cell.Next, the foil is wrapped around and/or glued to a wire-shaped ortube-shaped elongated core. The advantage of such a fabrication methodis that material that cannot withstand the deposition temperature of theabsorber layer, window layer, i-layer or transparent conductive layer412 can be used as an inner core for the solar cell. This manufacturingprocess can be used to manufacture any of the solar cells 402 disclosedin the present invention, where the conductive core 402 comprises aninner core and an outer conductive core. The inner core is anyconductive or nonconductive material disclosed herein whereas the outerconductive core is the web or foil onto which the absorber layer, windowlayer, and transparent conductive layer were deposited prior to rollingthe foil onto the inner core. In some embodiments, the web or foil isglued onto the inner core using appropriate glue.

An aspect of the present invention provides a method of manufacturing asolar cell comprising depositing an absorber layer on a first face of ametallic web or a conducting foil. Next, a window layer is deposited onto the absorber layer. Next, a transparent conductive layer is depositedon to the window layer. The metallic web or conducting foil is thenrolled around an elongated core, thereby forming an elongated solar cell402. In some embodiments, the absorber layer iscopper-indium-gallium-diselenide (Cu(InGa)Se₂) and the window layer iscadmium sulfide. In some embodiments, the metallic web is apolyimide/molybdenum web. In some embodiments, the conducting foil issteel foil or aluminum foil. In some embodiments, the elongated core ismade of a conductive metal, a metal-filled epoxy, a metal-filled glass,a metal-filled resin, or a conductive plastic.

In some embodiments, a transparent conducting oxide conductive film isdeposited on a tubular shaped or rigid solid rod shaped core rather thanwrapping a metal web or foil around the elongated core. In suchembodiments, the tubular shaped or rigid solid rod shaped core can be,for example, a plastic rod, a glass rod, a glass tube, or a plastictube. Such embodiments require some form of conductor in electricalcommunication with the interior face or back contact of thesemiconductor junction. In some embodiments, divots in the tubularshaped or rigid solid rod shaped elongated core are filled with aconductive metal in order to provide such a conductor. The conductor canbe inserted in the divots prior to depositing the transparent conductivelayer or conductive back contact film onto the tubular shaped or rigidsolid rod shaped elongated core. In some embodiments such a conductor isformed from a metal source that runs lengthwise along the side of theelongated solar cell 402. This metal can be deposited by evaporation,sputtering, screen printing, inkjet printing, metal pressing, conductiveink or glue used to attach a metal wire, or other means of metaldeposition.

More specific embodiments will now be disclosed. In some embodiments,the elongated core is a glass tubing having a divot that runs lengthwiseon the outer surface of the glass tubing, and the manufacturing methodcomprises depositing a conductor in the divot prior to the rolling step.In some embodiments, the glass tubing has a second divot that runslengthwise on the surface of the glass tubing. In such embodiments, thefirst divot and the second divot are on approximate or exact oppositecircumferential sides of the glass tubing. In such embodiments,accordingly, the method further comprises depositing a conductor in thesecond divot prior to the rolling or, in embodiments in which rolling isnot used, prior to the deposition of an inner transparent conductivelayer or conductive film, junction, and outer transparent conductivelayer onto the elongated core.

In some embodiments, the elongated core is a glass rod having a firstdivot that runs lengthwise on the surface of the glass rod and themethod comprises depositing a conductor in the first divot prior to therolling. In some embodiments, the glass rod has a second divot that runslengthwise on the surface of the glass rod and the first divot and thesecond divot are on approximate or exact opposite circumferential sidesof the glass rod. In such embodiments, accordingly, the method furthercomprises depositing a conductor in the second divot prior to therolling or, in embodiments in which rolling is not used, prior to thedeposition of an inner transparent conductive layer or conductive film,junction, and outer transparent conductive layer onto the elongatedcore. Suitable materials for the conductor are any of the materialsdescribed as a conductor herein including, but not limited to, aluminum,molybdenum, titanium, steel, nickel, silver, gold, or an alloy thereof.

FIG. 13 details a cross-section of a solar cell 402 in accordance withthe present invention. Solar cell 402 can be manufactured using eitherthe rolling method or deposition techniques. Components that havereference numerals corresponding to other embodiments of the presentinvention (e.g., 410, 412, and 420) are made of the same materialsdisclosed in such embodiments. In FIG. 13, there is an elongated tubing1306 having a first and second divot running lengthwise along the tubing(perpendicular to the plane of the page) that are on circumferentiallyopposing sides of tubing 1306 as illustrated. In typical embodiments,tubing 1306 is not conductive. For example, tubing 1306 is made ofplastic or glass in some embodiments. Conductive wiring 1302 is placedin the first and second divot as illustrated in FIG. 13. In someembodiments, the conductive wiring is made of any of the conductivematerials of the present invention. In some embodiments, conductivewiring 1302 is made out of aluminum, molybdenum, steel, nickel,titanium, silver, gold, or an alloy thereof. In embodiments where 1304is a conducting foil or metallic web, the conductive wiring 1302 isinserted into the divots prior to wrapping the metallic web orconducting foil 1304 around the elongated core 1306. In embodimentswhere 1304 is a transparent conductive oxide or conductive film, theconductive wiring 1302 is inserted into the divots prior to depositingthe transparent conductive oxide or conductive film 1304 onto elongatedcore 1306. As noted, in some embodiments the metallic web or conductingfoil 1304 is wrapped around tubing 1306. In some embodiments, metallicweb or conducting foil 1304 is glued to tubing 1306. In some embodimentslayer 1304 is not a metallic web or conducting foil. For instance, insome embodiments, layer 1304 is a transparent conductive layer. Such alayer is advantageous because it allow for thinner absorption layers inthe semiconductor junction. In embodiments where layer 1304 is atransparent conductive layer, the transparent conductive layer,semiconductor junction 410 and outer transparent conductive layer 412are deposited using deposition techniques.

One aspect of the invention provides a solar cell assembly comprising aplurality of elongated solar cells 402 each having the structuredisclosed in FIG. 13. That is, each elongated solar cell 402 in theplurality of elongated solar cells comprises an elongated tubing 1306, ametallic web or a conducting foil (or, alternatively, a layer of TCO)1304 circumferentially disposed on the elongated tubing 1306, asemiconductor junction 410 circumferentially disposed on the metallicweb or the conducting foil (or, alternatively, a layer of TCO) 1304 anda transparent conductive oxide layer 412 disposed on the semiconductorjunction 410. The elongated solar cells 402 in the plurality ofelongated solar cells are geometrically arranged in a parallel or a nearparallel manner thereby forming a planar array having a first face and asecond face. The plurality of elongated solar cells is arranged suchthat one or more elongated solar cells in the plurality of elongatedsolar cells are not in electrically conductive contact with adjacentelongated solar cells. In some embodiments, the elongated solar cellscan be in physical contact with each other if there is an insulativelayer between adjacent elongated solar cells. The solar cell assemblyfurther comprises a plurality of metal counter-electrodes. Eachrespective elongated solar cell 402 in the plurality of elongated solarcells is bound to a first corresponding metal counter-electrode 420 inthe plurality of metal counter-electrodes such that the first metalcounter-electrode lies in a first groove that runs lengthwise on therespective elongated solar cell 402. The apparatus further comprises atransparent electrically insulating substrate that covers all or aportion of the face of the planar array. A first and second elongatedsolar cell in the plurality of elongated solar cells are electricallyconnected in series by an electrical contact that connects the firstelectrode of the first elongated solar cell to the first correspondingcounter-electrode of the second elongated solar cell. In someembodiments, the elongated tubing 1306 is glass tubing or plastic tubinghaving a one or more grooves filled with a conductor 1302. In someembodiments, each respective elongated solar cell 402 in the pluralityof elongated solar cells is bound to a second corresponding metalcounter-electrode 420 in the plurality of metal counter-electrodes suchthat the second metal counter-electrode lies in a second groove thatruns lengthwise on the respective elongated solar cell 402 and such thatthe first groove and the second groove are on opposite or substantiallyopposite circumferential sides of the respective elongated solar cell402. In some embodiments, the plurality of elongated solar cells 402 isconfigured to receive direct light from the first face and the secondface of the planar array.

5.7 Static Concentrators

Encapsulated solar cell unit 300 may be assembled into bifacial arraysas, for example, any of assemblies 400 (FIG. 4), 700 (FIG. 7), 800 (FIG.8), 900 (FIG. 9), or 1000 (FIG. 10). In some embodiments, staticconcentrators are used to improve the performance of the solar cellassemblies of the present invention. The use of a static concentrator inone exemplary embodiment is illustrated in FIG. 11, where staticconcentrator 1102, with aperture AB, is used to increase the efficiencyof bifacial solar cell assembly CD, where solar cell assembly CD is, forexample, any of assemblies 400 (FIG. 4), 700 (FIG. 7), 800 (FIG. 8), 900(FIG. 9), or 1000 (FIG. 10) of other assemblies of solar cell units 300of the present invention. Static concentrator 1102 can be formed fromany static concentrator materials known in the art such as, for example,a simple, properly bent or molded aluminum sheet, or reflector film onpolyurethane. Concentrator 1102 is an example of a low concentrationratio, nonimaging, compound parabolic concentrator (CPC)-type collector.Any (CPC)-type collector can be used with the solar cell assemblies ofthe present invention. For more information on (CPC)-type collectors,see Pereira and Gordon, 1989, Journal of Solar Energy Engineering, 111,pp. 111-116, which is hereby incorporated by reference herein in itsentirety.

Additional static concentrators that can be used with the presentinvention are disclosed in Uematsu et al., 1999, Proceedings of the11^(th) International Photovoltaic Science and Engineering Conference,Sapporo, Japan, pp. 957-958; Uematsu et al., 1998, Proceedings of theSecond World Conference on Photovoltaic Solar Energy Conversion, Vienna,Austria, pp. 1570-1573; Warabisako et al., 1998, Proceedings of theSecond World Conference on Photovoltaic Solar Energy Conversion, Vienna,Austria, pp. 1226-1231; Eames et al., 1998, Proceedings of the SecondWorld Conference on Photovoltaic Solar Energy Conversion, ViennaAustria, pp. 2206-2209; Bowden et al., 1993, Proceedings of the 23^(rd)IEEE Photovoltaic Specialists Conference, pp. 1068-1072; and Parada etal., 1991, Proceedings of the 10^(th) EC Photovoltaic Solar EnergyConference, pp. 975-978, each of which is hereby incorporated byreference herein in its entirety.

In some embodiments, a static concentrator as illustrated in FIG. 12 isused. The bifacial solar cells illustrated in FIG. 12 can be anybifacial solar cell assembly of the present invention including. but notlimited to assembly 400 (FIG. 4), 700 (FIG. 7), 800 (FIG. 8), 900 (FIG.9), or 1000 (FIG. 10). The static concentrator illustrated in FIG. 12uses two sheets of cover glass on the front and rear of the module withsubmillimeter V-grooves that are designed to capture and reflectincident light as illustrated in the figure. More details of suchconcentrators are found in Uematsu et al., 2001, Solar Energy Materials& Solar Cell 67, 425-434 and Uematsu et al., 2001, Solar EnergyMaterials & Solar Cell 67, 441-448, each of which is hereby incorporatedby reference herein in its entirety. Additional static concentratorsthat can be used with the present invention are discussed in Handbook ofPhotovoltaic Science and Engineering, 2003, Luque and Hegedus (eds.),Wiley & Sons, West Sussex, England, Chapter 12, which is herebyincorporated by reference herein in its entirety.

5.8 Internal Reflector Embodiments

After elongated solar cells 402 are encapsulated as depicted, forexample, in FIG. 15, they may be arranged to form solar cell assemblies.FIG. 16 illustrates a solar cell assembly 1600 in accordance with thepresent invention. In this exemplary embodiment, an internal reflector1404 is used to enhance solar input into the solar cell system. As shownin FIG. 16, elongated solar cells 402 and an internal reflector 1404 areassembled into an alternating array as shown. Elongated solar cells 402in solar cell assembly 1600 have counter-electrodes 420 and electrodes440. As illustrated in FIG. 16, solar cell assembly 1600 comprises aplurality of elongated solar cells 402. There is no limit to the numberof solar cells 402 in this plurality (e.g., 10 or more, 100 or more,1000 or more, 10,000 or more, between 5,000 and one million solar cells402, etc.). Accordingly, solar cell assembly 1600 also comprises aplurality of internal reflectors 1404. There is no limit to the numberof internal reflectors 1404 in this plurality (e.g., 10 or more, 100 ormore, 1000 or more, 10,000 or more, between 5,000 and one millionreflector 1404, etc.).

Within solar cell assembly 1600, internal reflectors 1404 run lengthwisealong corresponding elongated solar cells 402. In some embodiments,internal reflectors 1404 have a hollow core. As in the case of elongatedconductive core 404, a hollow nonconductive core (e.g. substrate 403 ofFIG. 3B) is advantageous in many instances because it reduces the amountof material needed to make such devices, thereby lowering costs. In someembodiments, internal reflector 1404 is a plastic casing with a layer ofhighly reflective material (e.g., polished aluminum, aluminum alloy,silver, nickel, steel, etc.) deposited on the plastic casing. In someembodiments, internal reflector 1404 is a single piece made out ofpolished aluminum, aluminum alloy, silver, nickel, steel, etc. In someembodiments, internal reflector 1404 is a metal or plastic casing ontowhich is layered a metal foil tape. Exemplary metal foil tapes include,but are not limited to, 3M aluminum foil tape 425, 3M aluminum foil tape427, 3M aluminum foil tape 431, and 3M aluminum foil tape 439 (3M, St.Paul, Minn.). Internal reflector 1404 can adopt a broad range ofdesigns, only one of which is illustrated in FIG. 16. Central to thedesign of reflectors 1404 found in a preferred embodiment of the presentinvention is the desire to reflect direct light that enters into bothsides of solar cell assembly 1600 (i.e., side 1620 and side 1640).

In general, reflectors 1404 of the present invention are designed tooptimize reflection of light into adjacent elongated solar cells 402.Direct light that enters one side of solar cell assembly 1600 (e.g.,side 1940, above the plane of the solar cell assembly drawn in FIG. 16)is directly from the sun whereas light that enters the other side of thesolar cell (e.g., side 1620, below the plane of the solar cell assemblydrawn in FIG. 16) will have been reflected off of a surface. In someembodiments, this surface is Lambertian, a diffuse or an involutereflector. Thus, because each side of the solar cell assembly faces adifferent light environment, the shape of internal reflector 1404 onside 1620 may be different than on side 1640.

Although internal reflector 1404 is illustrated in FIG. 16 as having asymmetrical four-sided cross-sectional shape, the cross-sectional shapeof the internal reflectors 1404 of the present invention are not limitedto such a configuration. In some embodiments, a cross-sectional shape ofan internal reflector 1404 is astroid. In some embodiments, across-sectional shape of an internal reflector 1404 is four-sided and atleast one side of the four-sided cross-sectional shape is linear. Insome embodiments, a cross-sectional shape of an internal reflector 1404is four-sided and at least one side of the four-sided cross-sectionalshape is parabolic. In some embodiments, a cross-sectional shape of aninternal reflector 1404 is four-sided and at least one side of thefour-sided cross-sectional shape is concave. In some embodiments, across-sectional shape of an internal reflector 1404 is four-sided; andat least one side of the four-sided cross-sectional shape is circular orelliptical. In some embodiments, a cross-sectional shape of an internalreflector in the plurality of internal reflectors is four-sided and atleast one side of the four-sided cross-sectional shape defines a diffusesurface on the internal reflector. In some embodiments, across-sectional shape of an internal reflector 1404 is four-sided and atleast one side of the four-sided cross-sectional shape is the involuteof a cross-sectional shape of an elongated solar cell 402. In someembodiments, a cross-sectional shape of an internal reflector 1404 istwo-sided, three-sided, four-sided, five-sided, or six-sided. In someembodiments, a cross-sectional shape of an internal reflector in theplurality of internal reflectors 1404 is four-sided and at least oneside of the four-sided cross-sectional shape is faceted.

Additional features are added to reflectors 1404 to enhance thereflection onto adjacent elongated solar cells 402 in some embodiments.Modified reflectors 1404 are equipped with a strong reflective propertysuch that incident light is effectively reflected off the side surfaces1610 of the reflectors 1404. In some embodiments, the reflected lightoff surfaces 1610 does not have directional preference. In otherembodiments, the reflector surfaces 1610 are designed such that thereflected light is directed towards the elongated solar cell 402 foroptimal absorbance.

In some embodiments, the connection between an internal reflector 1404and an adjacent elongated solar cell is provided by an additionaladaptor piece. Such an adapter piece has surface features that arecomplementary to both the shapes of internal reflectors 1404 as well aselongated solar cells 402 in order to provide a tight fit between suchcomponents. In some embodiments, such adaptor pieces are fixed oninternal reflectors 1404. In other embodiments, the adaptor pieces arefixed on elongated solar cells 402. In additional embodiments, theconnection between elongated solar cells 402 and reflectors 1404 may bestrengthened by electrically conducting glue or tapes.

Diffuse Reflection. In some embodiments in accordance with the presentinvention, the side surface 1610 of reflector 1404 is a diffusereflecting surface (e.g., 1610 in FIG. 16). The concept of diffusereflection can be better appreciated with a first understanding ofspecular reflection. Specular reflection is defined as the reflectionoff smooth surfaces such as mirrors or a calm body of water (e.g., 1702in FIG. 17A). On a specular surface, light is reflected mainly in thedirection of the reflected ray and is attenuated by an amount dependentupon the physical properties of the surface. Since the light reflectedfrom the surface is mainly in the direction of the reflected ray, theposition of the observer (e.g., the position of the elongated solarcells 402) determines the perceived illumination of the surface.Specular reflection models the light reflecting properties of shiny ormirror-like surfaces. In contrast to specular reflection, reflection offrough surfaces such as clothing, paper, and the asphalt roadway leads toa different type of reflection known as diffuse reflection (FIG. 17B).Light incident on a diffuse reflection surface is reflected equally inall directions and is attenuated by an amount dependent upon thephysical properties of the surface. Since light is reflected equally inall directions the perceived illumination of the surface is notdependent on the position of the observer or receiver of the reflectedlight (e.g. the position of the elongated solar cell 402). Diffusereflection models the light reflecting properties of matt surfaces.

Diffuse reflection surfaces reflect off light with no directionaldependence for the viewer. Whether the surface is microscopically roughor smooth has a tremendous impact upon the subsequent reflection of abeam of light. Input light from a single directional source is reflectedoff in all directions on a diffuse reflecting surface (e.g., 1704 inFIG. 17B). Diffuse reflection originates from a combination of internalscattering of light, e.g., the light is absorbed and then re-emitted,and external scattering from the rough surface of the object.

Lambertian reflection. In some embodiments in accordance with thepresent invention, surface 1610 of reflector 1404 is a Lambertianreflecting surface (e.g., 1706 in FIG. 17C). A Lambertian source isdefined as an optical source that obeys Lambert's cosine law, i.e., thathas an intensity directly proportional to the cosine of the angle fromwhich it is viewed (FIG. 17C). Accordingly, a Lambertian surface isdefined as a surface that provides uniform diffusion of incidentradiation such that its radiance (or luminance) is the same in alldirections from which it can be measured (e.g., radiance is independentof viewing angle) with the caveat that the total area of the radiatingsurface is larger than the area being measured.

On a perfectly diffusing surface, the intensity of the light emanatingin a given direction from any small surface component is proportional tothe cosine of the angle of the normal to the surface. The brightness(luminance, radiance) of a Lambertian surface is constant regardless ofthe angle from which it is viewed.

The incident light {right arrow over (l)} strikes a Lambertian surface(FIG. 17C) and reflects in different directions. When the intensity of{right arrow over (l)} is defined as I_(in), the intensity (e.g.,I_(out)) of a reflected light{right arrow over (v)} can be defined asfollowing in accordance to Lambert's cosine law:${I_{out}\left( \overset{\rightarrow}{v} \right)} = {{I_{in}\left( \overset{\rightarrow}{l} \right)}{\varphi\left( {\overset{\rightarrow}{v},\overset{\rightarrow}{l}} \right)}\frac{\cos\quad\theta_{in}}{\cos\quad\theta_{out}}}$where φ({right arrow over (v)},{right arrow over (l)})=k_(d) cos θ_(out)and k_(d) is related to the surface property. The incident angle isdefined as θ_(in), and the reflected angle is defined as θ_(out). Usingthe vector dot product formula, the intensity of the reflected light canalso be written as:I _(out)({right arrow over (v)})= k _(d) I _(in)({right arrow over (l)}){right arrow over (l)}·{right arrow over (n)},where {right arrow over (n)} denotes a vector that is normal to theLambertian surface.

Such a Lambertian surface does not lose any incident light radiation,but re-emits it in all the available solid angles with a 2π radians, onthe illuminated side of the surface. Moreover, a Lambertian surfaceemits light so that the surface appears equally bright from anydirection. That is, equal projected areas radiate equal amounts ofluminous flux. Though this is an ideal, many real surfaces approach it.For example, a Lambertian surface can be created with a layer of diffusewhite paint. The reflectance of such a typical Lambertian surface may be93%. In some embodiments, the reflectance of a Lambertian surface may behigher than 93%. In some embodiments, the reflectance of a Lambertiansurface may be lower than 93%. Lambertian surfaces have been widely usedin LED design to provide optimized illumination, for example in U.S.Pat. No. 6,257,737 to Marshall, et al.; U.S. Pat. No. 6,661,521 toStern; and U.S. Pat. No. 6,603,243 to Parkyn, et al., which are herebyincorporated by reference in their entireties.

Advantageously, Lambertian surfaces 1610 on reflector 1404 effectivelyreflect light in all directions. The reflected light is then directedtowards the elongated solar cell 402 to enhance solar cell performance.

Reflection on involute surfaces. In some embodiments in accordance withthe present invention, surface 1610 of the reflector 1404 is an involutesurface of the elongated solar cell tube 402. In some embodiments, theelongated solar cell tube 402 is circular or near circular. Reflectorsurface 1610 is preferably the involute of a circle (e.g. 1804 in FIG.18A). The involute of circle 1802 is defined as the path traced out by apoint on a straight line that rolls around a circle. For example, theinvolute of a circle can be drawn in the following steps. First, attacha string to a point on a curve. Second, extend the string so that it istangent to the curve at the point of attachment. Third, wind the stringup, keeping it always taut. The locus of points traced out by the end ofthe string (e.g. 1804 in FIG. 18) is called the involute of the originalcircle 1802. The original circle 1802 is called the evolute of itsinvolute curve 1804.

Although in general a curve has a unique evolute, it has infinitely manyinvolutes corresponding to different choices of initial point. Aninvolute can also be thought of as any curve orthogonal to all thetangents to a given curve. For a circle of radius r, at any time t, itsequation can be written as:x=r cos ty=r sin tCorrespondingly, the parametric equation of the involute of the circleis:x _(i) =r(cos t+t sin t)y _(i=r)(sin t−t cos t)Evolute and involute are reciprocal functions. The evolute of aninvolute of a circle is a circle.

Involute surfaces have been implemented in numerous patent designs tooptimize light reflections. For example, a flash lamp reflector (U.S.Pat. No. 4,641,315 to Draggoo, hereby incorporated by reference hereinin its entirety) and concave light reflector devices (U.S. Pat. No.4,641,315 to Rose, hereby incorporated by reference herein in itsentirety), which are hereby incorporated by reference in theirentireties, both utilize involute surfaces to enhance light reflectionefficiency.

In FIG. 18B, an internal reflector 1404 is connected to two elongatedsolar cells 402. Details of both reflector 1404 and solar cell 402 areomitted to highlight the intrinsic relationship between the shapes ofthe elongated solar cell 402 and the shape of the side surface 1610 ofthe internal reflector 1404. Side surfaces 1610 are constructed suchthat they are the involute of the circular elongated solar cell 402.

Advantageously, the involute-evolute design imposes optimal interactionsbetween the side surfaces 1610 of reflectors 1404 and the adjacentelongated solar cell 402. When the side surface 1610 of the reflector1404 is an involute surface corresponding to the elongated solar cell402 that is adjacent or attached to the reflector 1404, light reflectseffectively off the involute surface in a direction that is optimizedtowards the elongated solar cell 402.

In some embodiments not illustrated in FIG. 16, elongated solar cells402 are swaged at their ends such that the diameter at the ends is lessthan the diameter towards the center of such cells. Electrodes 440 areplaced on these swaged ends.

Solar Cell Assembly. As illustrated in FIG. 16, solar cells in theplurality of elongated solar cells 402 are geometrically arranged in aparallel or near parallel manner. In some embodiments, elongatedconductive core 404 is any of the dual layer cores described in Section5.4. In some embodiments, rather forming a conductive core 404,back-electrode 404 is a thin layer of metal deposited on a substrate 403as illustrated, for example, in FIG. 3B. In some embodiments, theterminal ends of elongated solar cells 402 can be stripped down to theouter core. For example, consider the case in which elongated solar cell402 is constructed out of an inner core made of a cylindrical substrate403 and an outer core (back-electrode 404) made of molybdenum. In such acase, the end of elongated solar cell 402 can be stripped down to themolybdenum back-electrode 404 and electrode 440 can be electricallyconnected with back-electrode 404.

In some embodiments, each internal reflector 1404 connects to twoencapsulated elongated solar cells 402 (e.g., depicted as 300 in FIGS.15 and 16), for example, in the manner illustrated in FIG. 16. Becauseof this, elongated solar cells 402 are effectively joined into a singlecomposite device. In FIG. 16, electrodes 440 extend the connection fromback-electrode 404. In some embodiments, internal reflector units 1404are connected to encapsulated solar cells 300 via indentations ontransparent tubular casing 310. In some embodiments, the indentations ontransparent tubular casing 310 are created to complement the shape ofthe internal reflector unit 1404. Indentations on two transparenttubular casing 310 are used to lock in one internal reflector unit 1404that is positioned between the two encapsulated solar cells 300. In someembodiments, adhesive materials, e.g., epoxy glue, are used to fortifythe connections between the internal reflector unit 1404 and theadjacent encapsulated solar cell units 300 such that solar radiation isproperly reflected towards the encapsulated solar cell units 300 forabsorption.

In some embodiments in accordance with the present invention, internalreflector unit 1404 and transparent tubular casing 310 may be created inthe same molding process. For example, an array of alternatingtransparent tubular casing 310 and astroid reflectors 1404, e.g., shownas 1900 in FIG. 19, can be made as a single composite entity. Additionalmodifications may be done to enhance the albedo effect from the internalreflector unit 1404 or to promote better fitting between transparenttubular casing 310 and solar cell 402. The tubular casing 310 maycontain internal modifications that complement the shapes of someembodiments of the solar cell 402. There is no limit to the number ofinternal reflectors 1404 or tubular casing 310 in the assembly asdepicted in FIG. 19 (e.g., 10 or more, 100 or more, 1000 or more, 10,000or more, between 5,000 and one million internal reflectors 1404 andtubular casing 310, etc.).

6. REFERENCES CITED

All references cited herein are incorporated herein by reference intheir entirety and for all purposes to the same extent as if eachindividual publication or patent or patent application was specificallyand individually indicated to be incorporated by reference in itsentirety for all purposes.

Many modifications and variations of this invention can be made withoutdeparting from its spirit and scope, as will be apparent to thoseskilled in the art. The specific embodiments described herein areoffered by way of example only, and the invention is to be limited onlyby the terms of the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. A solar cell unit comprising: (A) a cylindrical shaped solar cell,said cylindrical shaped solar cell comprising: a rigid substrate that iseither (i) tubular or (ii) a solid rod; a back-electrodecircumferentially disposed on the substrate; a semiconductor junctionlayer circumferentially disposed on said back-electrode; and atransparent conductive layer circumferentially disposed on saidsemiconductor junction; and (B) a transparent tubular casingcircumferentially disposed onto said cylindrical shaped solar cell. 2.The solar cell unit of claim 1, wherein the transparent tubular casingis made of plastic or glass.
 3. The solar cell unit of claim 1, whereinthe transparent tubular casing comprises aluminosilicate glass,borosilicate glass, dichroic glass, germanium/semiconductor glass, glassceramic, silicate/fused silica glass, soda lime glass, quartz glass,chalcogenide/sulphide glass, fluoride glass, flint glass, or cereatedglass.
 4. The solar cell unit of claim 1, wherein the transparenttubular casing comprises a urethane polymer, an acrylic polymer, afluoropolymer, a silicone, a silicone gel, an epoxy, a polyamide, or apolyolefin.
 5. The solar cell unit of claim 1, wherein the transparenttubular casing comprises polymethylmethacrylate (PMMA), poly-dimethylsiloxane (PDMS), ethylene vinyl acetate (EVA), perfluoroalkoxyfluorocarbon (PFA), nylon, cross-linked polyethylene (PEX),polypropylene (PP), polyethylene terephtalate glycol (PETG),polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), orpolyvinylidene fluoride (PVDF).
 6. The solar cell unit of claim 1,wherein the rigid substrate comprises plastic or glass.
 7. The solarcell unit of claim 1, wherein the rigid substrate comprises metal ormetal alloy.
 8. The solar cell unit of claim 1, wherein the rigidsubstrate comprises a urethane polymer, an acrylic polymer, afluoropolymer, polybenzamidazole, polymide, polytetrafluoroethylene,polyetheretherketone, polyamide-imide, a glass-based phenolic,polystyrene, cross-linked polystyrene, polyester, polycarbonate,polyethylene, acrylonitrile-butadiene-styrene, polytetrafluoro-ethylene,polymethacrylate, nylon 6,6, cellulose acetate butyrate, celluloseacetate, rigid vinyl, plasticized vinyl, or polypropylene.
 9. The solarcell unit of claim 1, wherein the rigid substrate comprisesaluminosilicate glass, borosilicate glass, dichroic glass,germanium/semiconductor glass, glass ceramic, silicate/fused silicaglass, soda lime glass, quartz glass, chalcogenide/sulphide glass,fluoride glass, a glass-based phenolic, flint glass, or cereated glass.10. The solar cell unit of claim 1, wherein the rigid substrate istubular shaped and a fluid is passed through said substrate.
 11. Thesolar cell unit of claim 10, wherein the fluid is air, water, nitrogen,or helium.
 12. The solar cell unit of claim 1, wherein the rigidsubstrate has a solid core.
 13. The solar cell unit of claim 1, whereinthe back-electrode is made of aluminum, molybdenum, tungsten, vanadium,rhodium, niobium, chromium, tantalum, titanium, steel, nickel, platinum,silver, gold, an alloy thereof, or any combination thereof.
 14. Thesolar cell unit of claim 1, wherein the back-electrode is made of indiumtin oxide, titanium nitride, tin oxide, fluorine doped tin oxide, dopedzinc oxide, aluminum doped zinc oxide, gallium doped zinc oxide, borondoped zinc oxide, indium-zinc oxide, a metal-carbon black-filled oxide,a graphite-carbon black-filled oxide, a carbon black-filled oxide, asuperconductive carbon black-filled oxide, an epoxy, a conductive glass,or a conductive plastic.
 15. The solar cell unit of claim 1, wherein thesemiconductor junction comprises a homojunction, a heterojunction, aheteroface junction, a buried homojunction, a p-i-n junction, or atandem junction.
 16. The solar cell unit of claim 1, wherein thetransparent conductive layer comprises carbon nanotubes, tin oxide,fluorine doped tin oxide, indium-tin oxide (ITO), doped zinc oxide,aluminum doped zinc oxide, gallium doped zinc oxide, boron doped zincoxides indium-zinc oxide or any combination thereof.
 17. The solar cellunit of claim 1, wherein said semiconductor junction comprises anabsorber layer and a junction partner layer, wherein said junctionpartner layer is circumferentially disposed on said absorber layer. 18.The solar cell unit of claim 17, wherein said absorber layer iscopper-indium-gallium-diselenide and said junction partner layer isIn₂Se₃, In₂S₃, ZnS, ZnSe, CdInS, CdZnS, ZnIn₂Se₄, Zn_(1-x)Mg_(x)O, CdS,SnO₂, ZnO, ZrO₂, or doped ZnO.
 19. The solar cell unit of claim 17,wherein said absorber layer is between 0.5 μm and 2.0 μm thick.
 20. Thesolar cell unit of claim 17, wherein a composition ratio of Cu/(In+Ga)in said absorber layer is between 0.7 and 0.95.
 21. The solar cell unitof claim 17, wherein a composition ratio of Ga/(In+Ga) in said absorberlayer is between 0.2 and 0.4.
 22. The solar cell unit of claim 17,wherein the absorber layer comprises CIGS having a <110>crystallographic orientation.
 23. The solar cell unit of claim 17,wherein the absorber layer comprises CIGS having a <112>crystallographic orientation.
 24. The solar cell unit of claim 17,wherein the absorber layer comprises CIGS that is randomly oriented. 25.The solar cell unit of claim 1, wherein the cylindrical shaped solarcell further comprises an intrinsic layer circumferentially disposed onsaid semiconductor junction and wherein the transparent conductive layeris disposed on said intrinsic layer.
 26. The solar cell unit of claim25, wherein the intrinsic layer comprises an undoped transparent oxide.27. The solar cell unit of claim 25, wherein the intrinsic layercomprises undoped zinc oxide.
 28. The solar cell unit of claim 1,further comprising a filler layer circumferentially disposed on saidtransparent conductive layer, wherein said transparent tubular casing iscircumferentially disposed on said filler layer therebycircumferentially sealing said cylindrical shaped solar cell.
 29. Thesolar cell unit of claim 28, wherein the filler layer comprises ethylenevinyl acetate (EVA), silicone, silicone gel, epoxy, polydimethylsiloxane (PDMS), RTV silicone rubber, polyvinyl butyral (PVB),thermoplastic polyurethane (TPU), a polycarbonate, an acrylic, afluoropolymer, or a urethane.
 30. The solar cell unit of claim 1,further comprising a water resistant layer circumferentially disposed onsaid transparent conductive layer, wherein said transparent tubularcasing is circumferentially disposed on said water resistant layerthereby circumferentially sealing said cylindrical shaped solar cell.31. The solar cell unit of claim 30, wherein the water resistant layercomprises clear silicone, SiN, SiO_(x)N_(y), SiO_(x), or Al₂O₃, where xand y are integers.
 32. The solar cell unit of claim 1, furthercomprising: a water resistant layer circumferentially disposed on saidtransparent conductive layer; and a filler layer circumferentiallydisposed on said water resistant layer, wherein said transparent tubularcasing is circumferentially disposed on said filler layer therebycircumferentially sealing said cylindrical shaped solar cell.
 33. Thesolar cell unit of claim 1, further comprising: a filler layercircumferentially disposed on said transparent conductive layer; and awater resistant layer circumferentially disposed on said water resistantlayer, wherein said transparent tubular casing is circumferentiallydisposed on said water resistant layer thereby circumferentially sealingsaid cylindrical shaped solar cell.
 34. The solar cell unit of claim 1,further comprising an antireflective coating circumferentially disposedon said transparent tubular casing.
 35. The solar cell unit of claim 34,wherein the antireflective coating comprises MgF₂, silicon nitrate,titanium nitrate, silicon monoxide, or silicon oxide nitrite.
 36. Thesolar cell unit of claim 1, wherein said cylindrical shaped solar cellhas a cylindrical axis, said cylindrical shaped solar cell furthercomprising at least one electrode strip, wherein each electrode strip inthe at least one electrode strip is overlayed on the transparentconductive layer along the cylindrical axis of the solar cell.
 37. Thesolar cell unit of claim 36, wherein the at least one electrode stripcomprises a plurality of electrode strips that are positioned at spacedintervals on the transparent conductive layer such that the plurality ofelectrode strips run parallel or approximately parallel to each otheralong the cylindrical axis of the solar cell.
 38. The solar cell unit ofclaim 37, wherein electrode strips in the plurality of electrode stripsare spaced out at sixty degree intervals on a surface of the transparentconductive layer.
 39. The solar cell unit of claim 37, wherein electrodestrips in the plurality of electrode strips are spaced out at evenintervals on a surface of the transparent conductive layer.
 40. Thesolar cell unit of claim 37, wherein electrode strips in the pluralityof electrode strips are spaced out at uneven intervals on a surface ofthe transparent conductive layer.
 41. The solar cell unit of claim 36,wherein the at least one electrode strip comprises a conductive epoxy, aconductive ink, copper or an alloy thereof, aluminum or an alloythereof, nickel or an alloy thereof, silver or an alloy thereof, gold oran alloy thereof, a conductive glue, or a conductive plastic.
 42. Thesolar cell unit of claim 36, wherein the at least one electrode stripcomprises a plurality of electrode strips overlayed on the transparentconductive layer along the cylindrical axis of the solar cell; andwherein said cylindrical shaped solar cell further comprises at leastone grid line that electrically connects a first electrode strip and asecond electrode strip in the plurality of electrode strips.
 43. Thesolar cell unit of claim 1, wherein a length of said cylindrical shapedsolar cell is between 2 centimeters and 300 centimeters.
 44. The solarcell unit of claim 1, wherein a length of said cylindrical shaped solarcell is between 2 centimeters and 30 centimeters.
 45. The solar cellunit of claim 1, wherein a length of said cylindrical shaped solar cellis between 30 centimeters and 300 centimeters.
 46. A solar cell assemblycomprising a plurality of solar cell units, each solar cell unit in theplurality of solar cell units having the structure of the solar cellunit of claim 1, wherein solar cell units in said plurality of solarcell units are arranged in coplanar rows to form said solar cellassembly.
 47. The solar cell assembly of claim 46, further comprising analbedo surface positioned to reflect sunlight into the plurality ofsolar cell units.
 48. The solar cell assembly of claim 47, wherein thealbedo surface has an albedo that exceeds 80%.
 49. The solar cellassembly of claim 47, wherein the albedo surface is Lambertian ordiffuse.
 50. The solar cell assembly of claim 46, wherein a first solarcell unit and a second solar cell unit in the plurality of solar cellunits is electrically arranged in series.
 51. The solar cell assembly ofclaim 46, wherein a first solar cell unit and a second solar cell unitin the plurality of solar cell units is electrically arranged inparallel.
 52. The solar cell unit of claim 1, wherein an outer surfaceof the transparent tubular casing is textured.
 53. A solar cell assemblycomprising: a plurality of solar cell units, each solar cell unit in theplurality of solar cell units having the structure of the solar cellunit of claim 1; and a plurality of internal reflectors, wherein theplurality of solar cell units and the plurality of internal reflectorsare arranged in coplanar rows in which internal reflectors in theplurality of solar cell units abut solar cell units in the plurality ofsolar cell units thereby forming the solar cell assembly.
 54. The solarcell assembly of claim 53, wherein an internal reflector in saidplurality of internal reflectors has a hollow core.
 55. The solar cellassembly of claim 53, wherein an internal reflector in said plurality ofinternal reflectors comprises a plastic casing with a layer ofreflective material deposited on said plastic casing.
 56. The solar cellassembly of claim 55, wherein the layer of reflective material ispolished aluminum, aluminum alloy, silver, nickel or steel.
 57. Thesolar cell assembly of claim 53, wherein an internal reflector in saidplurality of internal reflectors is a single piece made out of areflective material.
 58. The solar cell assembly of claim 57, whereinthe reflective material is polished aluminum, aluminum alloy, silver,nickel or steel.
 59. The solar cell assembly of claim 53, wherein aninternal reflector in said plurality of internal reflectors comprises aplastic casing onto which is layered a metal foil tape.
 60. The solarcell assembly of claim 59, wherein the metal foil tape is aluminum foiltape.
 61. The solar cell assembly of claim 53, wherein a cross-sectionalshape of an internal reflector in said plurality of internal reflectorsis asteroid or involute.
 62. The solar cell assembly of claim 53,wherein a cross-sectional shape of an internal reflector in saidplurality of internal reflectors is four-sided; and a side of saidfour-sided cross-sectional shape is linear, parabolic, concave, circularor elliptical.
 63. The solar cell assembly of claim 53, wherein across-sectional shape of an internal reflector in said plurality ofinternal reflectors is four-sided; and a side of said four-sidedcross-sectional shape defines a diffuse surface on said internalreflector.
 64. The solar cell assembly of claim 53, wherein a firstsolar cell unit and a second solar cell unit in the plurality of solarcell units is electrically arranged in series.
 65. The solar cellassembly of claim 53, wherein a first solar cell unit and a second solarcell unit in the plurality of solar cell units is electrically arrangedin parallel.
 66. A solar cell unit comprising: (A) a solar cellcomprising: a substrate, wherein the substrate is either (i) tubularshaped or (ii) rigid solid shaped; a back-electrode circumferentiallydisposed on said substrate; a semiconductor junction circumferentiallydisposed on said back-electrode; and a transparent conductive layerdisposed on said semiconductor junction; (B) a filler layercircumferentially disposed on said transparent conductive layer; and C)a transparent tubular casing circumferentially disposed onto said fillerlayer.
 67. The solar cell unit of claim 66, wherein said substrate has asolid core.
 68. The solar cell unit of claim 66, wherein the substratecomprises plastic, metal or glass.
 69. The solar cell unit of claim 66,wherein the substrate comprises a urethane polymer, an acrylic polymer,a fluoropolymer, polybenzamidazole, polymide, polytetrafluoroethylene,polyetheretherketone, polyamide-imide, glass-based phenolic,polystyrene, cross-linked polystyrene, polyester, polycarbonate,polyethylene, polyethylene, acrylonitrile-butadiene-styrene,polytetrafluoro-ethylene, polymethacrylate, nylon 6,6, cellulose acetatebutyrate, cellulose acetate, rigid vinyl, plasticized vinyl, orpolypropylene.
 70. The solar cell unit of claim 66, wherein thesubstrate comprises aluminosilicate glass, borosilicate glass, dichroicglass, germanium/semiconductor glass, glass ceramic, silicate/fusedsilica glass, soda lime glass, quartz glass, chalcogenide/sulphideglass, fluoride glass, a glass-based phenolic, flint glass, or cereatedglass.
 71. The solar cell unit of claim 66, wherein said semiconductorjunction comprises an absorber layer and a junction partner layer,wherein said junction partner layer is circumferentially disposed onsaid absorber layer; and said absorber layer is circumferentiallydisposed on said back-electrode.
 72. The solar cell unit of claim 71,wherein said absorber layer is copper-indium-gallium-diselenide and saidjunction partner layer is CdS, SnO₂, ZnO, ZrO₂, or doped ZnO.
 73. Thesolar cell unit of claim 71, wherein the absorber layer comprises CIGShaving a <110> crystallographic orientation. a <112> crystallographicorientation, or no crystallographic orientation.
 74. The solar cell unitof claim 66, wherein said solar cell unit further comprises: (D) anantireflective coating circumferentially disposed on said transparenttubular casing.
 75. The solar cell unit of claim 74, wherein theantireflective coating comprises MgF₂, silicone nitrate, titaniumnitrate, silicon monoxide, or silicone oxide nitrite.
 76. The solar cellunit of claim 66, wherein$r_{i} \geq \frac{r_{o}}{\eta_{{outer}\quad{ring}}}$ wherein r_(i) is aradius of the cylindrical shaped solar cell; r_(o) is the radius of thetransparent tubular casing; and η_(outer ring) is the refractive indexof the transparent tubular casing.
 77. The solar cell unit of claim 66,wherein the transparent tubular casing is formed directly on said fillerlayer by casting or UV curing.
 78. The solar cell unit of claim 66,wherein the transparent tubular casing comprises a plurality oftransparent tubular casing layers including a first transparent tubularcasing layer and a second transparent tubular casing layer, and whereinthe first transparent tubular casing layer is circumferentially disposedon said filler layer and the second transparent tubular casing layer iscircumferentially disposed on said first transparent tubular casinglayer.
 79. A solar cell unit comprising: (A) a solar cell comprising: asubstrate, wherein said substrate is either (i) tubular shaped or (ii)rigid solid rod shaped; a back-electrode circumferentially disposed onthe substrate; a semiconductor junction circumferentially disposed onthe back-electrode; and a transparent conductive layer disposed on thesemiconductor junction; (B) a water resistant layer circumferentiallydisposed on the transparent conductive layer; (C) a filler layercircumferentially disposed on the water resistant layer; and (D) atransparent tubular casing circumferentially disposed on the fillerlayer.
 80. The solar cell unit of claim 79, wherein said substrate is atube.
 81. The solar cell unit of claim 79, wherein$r_{i} \geq \frac{r_{o}}{\eta_{{outer}\quad{ring}}}$ wherein r_(i) is aradius of the cylindrical shaped solar cell; r_(o) is the radius of thetransparent tubular casing; and η_(outer ring) is the refractive indexof the transparent tubular casing.
 82. The solar cell unit of claim 79,wherein the transparent tubular casing comprises a plurality oftransparent tubular casing layers including a first transparent tubularcasing layer and a second transparent tubular casing layer, and whereinthe first transparent tubular casing layer is circumferentially disposedon said filler layer and the second transparent tubular casing layer iscircumferentially disposed on said first transparent tubular casinglayer.
 83. The solar cell unit of claim 79, wherein the transparenttubular casing is formed by casting or UV curing directly onto thefiller layer.
 84. A solar cell unit comprising: (A) a solar cellcomprising: a substrate, wherein said substrate is tubular shaped orrigid solid rod shaped; a back-electrode circumferentially disposed onsaid substrate; a semiconductor junction circumferentially disposed onsaid back-electrode; and a transparent conductive layer disposed on saidsemiconductor junction; (B) a filler layer circumferentially disposed onsaid transparent conductive layer; and (C) a water resistant layercircumferentially disposed on said filler layer; and (D) a transparenttubular casing circumferentially disposed onto said water resistantlayer.
 85. The solar cell unit of claim 84, wherein$r_{i} \geq \frac{r_{o}}{\eta_{{outer}\quad{ring}}}$ wherein r_(i) is aradius of the cylindrical shaped solar cell; r_(o) is the radius of thetransparent tubular casing; and η_(outer ring) is the refractive indexof the transparent tubular casing.
 86. The solar cell unit of claim 84,wherein said substrate is a tube.
 87. The solar cell unit of claim 84,wherein the transparent tubular casing is formed by casting or UV curingdirectly onto the water resistant layer.
 88. The solar cell unit ofclaim 1, wherein $r_{i} \geq \frac{r_{o}}{\eta_{{outer}\quad{ring}}}$wherein r_(i) is a radius of the cylindrical shaped solar cell; r_(o) isthe radius of the transparent tubular casing; and η_(outer ring) is therefractive index of the transparent tubular casing.
 89. The solar cellunit of claim 1, wherein the transparent tubular casing is formeddirectly on said cylindrical shaped solar cell by casting or UV curing.90. The solar cell unit of claim 1, wherein the transparent tubularcasing comprises a plurality of transparent tubular casing layersincluding a first transparent tubular casing layer and a secondtransparent tubular casing layer, and wherein the first transparenttubular casing layer is circumferentially disposed on said semiconductorjunction and the second transparent tubular casing layer iscircumferentially disposed on said first transparent tubular casinglayer.
 91. The solar cell unit of claim 1, wherein the transparentconductive layer is coated with a fluorescent material.
 92. The solarcell unit of claim 1, wherein a luminal or an exterior surface of saidtransparent tubular casing is coated with a fluorescent material. 93.The solar cell unit of claim 32, wherein the water resistant layer orthe filler layer is coated with a fluorescent material.
 94. The solarcell unit of claim 33, wherein the filler layer or the water resistantlayer is coated with a fluorescent material.
 95. The solar cell unit ofclaim 1, wherein the rigid substrate is a plastic rod.
 96. The solarcell unit of claim 1, wherein the rigid substrate is a glass rod. 97.The solar cell unit of claim 1, wherein the rigid substrate is a glasstube.
 98. The solar cell unit of claim 1, wherein the substrate is aplastic tube.